Member of the religious caste and the family of Mir, Delenn was brought up by her father after her mother joined a religious order called the Sisters of Valeria. In fact, Delenn only saw her mother twice during her childhood. Her family descents from Valen and thus have some human DNA in their bloodline.

Delenn joined the temple at a young age and when she was a mere acolyte, she was chosen by Dukhat, the great Minbari leader, to be his assistant and protégée. Guided and taught by Dukhat, Delenn herself eventually became a member of the Grey Council. When she joined the Council the triluminary used in the ceremony began to glow - it had been programmed for Sinclair/Valen's DNA and recognised her as his descendant. Just hours after the ceremony the Council's ship made the first contact with humans. The humans misinterpreted the ship's open gunports - a sign of respect - and opened fire. In the following battle many Council members, including Dukhat, were killed. The remaining Council was divided whether to revenge or return to Minbar and Delenn's vote was the deciding one. Grief-stricken she ordered the warrior caste to destroy the entire human race. After calming down she regretted her decision, but by then it was already too late and the holy war had already began.

During the war Delenn found out that Dukhat had been working with the Vorlons and that humans would have a crucial role in the next Shadow War. In the light of her discoveries Delenn, with the help of Lennon, the Ranger One, began to pursue a peaceful solution to the war. She sent Lennon to meet with the human representatives, John Sheridan and Stephen Franklin, and to start peace-negotiations. Unfortunately the Centauri mistook the meeting as a weapons transaction between the humans and the Narn. This resulted an attack where Lennon was killed and his human counterparts were stranded on a deserted planet and later captured by the Minbari. Delenn, like the other Minbari blamed the humans for Lennon's death. It was only the password 'Isil'zha' (meaning 'future' and referring to the Vorlons) taught to Sheridan by Lennon that convinced her of their innocence. To the astonishment of the other Minbari, she let the prisoners go unharmed.

Delenn was also present when the war came to an end. After even the last hope of peace was gone, during the so-called Battle of the Line, she ordered one of the human soldiers, who happened to be Jeffrey Sinclair, to be captured and interrogated in order to find out about Earth's defenses. During the interrogations the Grey Council found out that the Minbari were reborn as humans. Because a Minbari cannot kill another Minbari, they immediately gave the order to surrender.

When the humans and the Minbari began their co-operation on the Babylon Project, Delenn became the Minbari ambassador for Babylon 5 even though she was a somewhat high-ranking official for such a lowly position. Her assignment was mainly just a smokescreen, though. Her real task was to keep an eye on Sinclair, now the commander of B5, and to make sure he didn't remember what had really happened during the Battle of the Line.

In 2258 Delenn was offered the leadership of the Grey Council, but she turned it down, believing that she was more needed on B5 than on Minbar. Together with Kosh Naranek, whom she had first met during the Earth-Minbari War, Delenn began to prepare for the Shadow War. One of their allies was Delenn's old teacher Draal, who took over the task of maintaining the Great Machine on Epsilon 3 and thus brought a powerful weapon at the disposal of the constantly growing Army of Light. Still, despite of all her her allies Delenn was alone (and acting against the orders of the Grey Council) in her ordeal in late 2258 when, in order to fulfill one of Valen's prophesies, she used the triluminary to enter a cocoon and metamorphose into a half human, half Minbari. Officially the reason for her transformation was to build a bridge between the humans and the Minbari, but the real reason was to restore the balance hat had been disturbed when Sinclair a thousand years earlier had become Valen.

Delenn's transformation wasn't accepted by all of the Minbari, and because she had disobeyed the Council's orders, she was removed from it. Humans also despised her new appearance and considered her to be a disgrace to the memory of the humans who had died in the Minbari War. One of the few people who immediately accepted her was Captain Sheridan and in time the friendship between the two of them became love. For a long time Delenn was able to keep the truth about the Shadows and the alliance against them a secret, but when Sheridan found out about Mr. Morden and the truth about the death of his wife Anna, she was forced to let him in on the alliance. Still, the Vorlons had their doubts about Delenn and called for the inquisitor Sebastian to question her. During the interrogation Sebastian tried to make her confess that she believed she was chosen by the gods, but she passed the test with her willingness to sacrifice her life for Sheridan.

In 2260, around the same time as B5 seceded from the Earth Alliance, Delenn (frustrated by the Council's indecisiveness, and again following Valen's prophesies) broke the Grey Council. She then took the White Star and led three Minbari battleships to B5 space just in time to save the station from Earth's attack troops. During the next few months Kosh was killed and Delenn herself only barely survived an attack by a vengeful Nightwatch member (she took a knife that was meant for Sheridan). Still, this could be called a relatively peaceful period of time compared to what was coming.

Already for some time Delenn had known the truth about the fate of Babylon 4 and when she received a letter from the past, she knew it was time to act. With the help of the Great Machine she - together with Marcus Cole, Sinclair, Commander Ivanova, Sheridan, Lennier and Zathras - travelled six years into the past, saved B4 from an attack by the shadow minions and then sent the station, Zathras and Sinclair a thousand years back in time. It was during this mission that Sheridan momentarily became 'unstuck in time' and visited a future where he and Delenn were married and had a son named David.

With Sinclair gone, the Rangers were left without a leader and Delenn reluctantly agreed to become the Ranger One. The Warrior Caste opposed her, claiming that they should have control over all military forces, including the Rangers. Neroon, who in 2259 had taken Delenn's place in the Grey Council travelled to B5 in order to prevent Delenn's inauguration with any means necessary. It was only Marcus' determination to protect Delenn even at the cost of his own life that changed Neroon's mind.

Because of her high position in the Army of Light, Delenn was forced to keep information from her friends and allies. She kept back many things including the truth about the Shadows' involvement in the destruction of Narn and the real number of the whitestars. All hell broke loose when Anna Sheridan returned from the dead in order to persuade her husband to join the Shadows. Delenn begged Sheridan to believe that she had not known that Anna was alive, only suspected. Nevertheless, she had to confess that had she known, she would have not told him anyway. Angry and heart-broken Sheridan followed Anna to Z'ha'dum and to his death.

Believing that she was to blame for Sheridan's fate, Delenn began a fast that was a part of the Minbari mourning rituals even though she knew that being no-longer a full Minbari, she would most likely not survive it. Fortunately Franklin was able to restore her will to live and she began to plan a direct attack against the Shadow homeworld.

Before she could bring her plan to life, though, Sheridan (with the help of Lorien) came back from the dead. Together they continued to gather the greatest fleet in history to fight not only the Shadows, but also the Vorlons, who had turned against their former allies. During the next few months the fleet became a reality and in the end Sheridan and Delenn (again with the help of Lorien) managed to drive the First Ones beyond the Rim and end the Great Wars for good.

Meanwhile on Earth President Clark's propaganda became more and more obviously directed at Delenn. The media portrayed her as a Minbari seductress who took advantage of Sheridan's 'mental instability' in order to conduct genetic experiments on humans. The Minbari didn't like Sheridan and Delenn's relationship either and she was called back home to justify her choice of mate to her clan. During a ceremony called the Dreaming Delenn remembered what Dukhat had told her just before his death - that she was a descendant of Valen, or more correctly Sinclair. In order to keep the 'impurity' of the Valen's descendants a secret, Delenn's clan allowed her to to marry Sheridan as a 'gesture of goodwill'.

In 2261 it became obvious that there would be a price on Delenn's earlier actions. With no Grey Council to balance the power between the castes, the disagreements between the Religious Caste and the Warrior Caste turned into a bloody civil war that broke the thousand years of peace. Even Delenn, who had always had faith in Valen's prophesies, began to doubt her actions and blame herself for the war. In order to restore the peace she then turned to Neroon for help. This alliance between two former enemies led to a misunderstanding that would have cost her life had it not been for Lennier who risked his own life to save her.

After arriving to Minbar Delenn contacted Shakiri, the leader of the warriors, and set up a meeting in an ancient temple. She announced that the Religious Caste would surrender, but refused the Warriors' proposition of a Warriors' Council, and demanded that the dominance of the castes should be decided 'by the old ways'. And with these words she challenged Shakiri to the ritual of the Starfire Wheel. The cowardly warrior didn't want to die for his caste, and only Delenn remained inside the wheel. She was prepared to sacrifice herself, like the ancient rules required, but at the last moment Neroon switched places with her, declaring that although he had been born in the warrior caste, the calling of his heart was religious. Neroon's sacrifice meant that the Religious Caste had won and Delenn had the authority to form a new Grey Council. Like Valen before her, she called the Council together, but instead of restoring the balance of power, she gave the majority of seats in it to the Worker Caste who had remained neutral during the war.

While Sheridan was trying to liberate Earth, Delenn was working hard to achieve intergalactic peace. Together with Londo Mollari, G'Kar and the League of Non-Aligned Worlds, she created the Interstellar Alliance (which the newly-freed Earth later joined) and became a member of its Advisory Board. With Earth now free and the Alliance taking its first steps, Sheridan (who had been elected as the President of the ISA) and Delenn returned to B5 and were married. Still, their marriage had a negative side to it as well - Delenn's faithful attaché Lennier (who had been secretly in love with her for years) could not stand the thought of her marrying someone else and left B5 to join the Rangers.

The Interstellar Alliance had barely left its crib when it was already at the brink of destruction. An unknown military force (the Drakh, disguised as the Centauri) began randomly attack the shipping routes, turning the member worlds against each other. For a while Sheridan managed to keep the quarreling governments off each other's throats, but as the attacks continued and the accusing fingers began to point towards the Centauri, Delenn began her own investigations. In secret from her husband she assigned Lennier to a whitestar patrolling near the Centauri space. Once there Lennier found evidence of Centauri involvement. This didn't restore peace to the Alliance, far from it. The arguments between the members only increased and in no time the Alliance was at war with the Centauri.

The war wasn't the only worry the Alliance had - the numerous wars and campaigns of the previous years had cost a large portion of the whitestar fleet. In order to maintain the military power of the Alliance, Delenn and Sheridan contacted their governments and persuaded them into an economic alliance to create a new, better class of whitestar ships. Because of this Delenn had to travel to Minbar in person, but before she got there she was attacked by the Centauri and it was only Londo's plea to the Drakh that saved her life. She was saved just in time to witness how this former friend and ally ascended to emperorhood and turned his people against the Alliance.

In the middle of all this sorrow there was also some joy as Delenn became pregnant. It was a joyous news even though Franklin suspected that her unique physique could cause complications.

In late 2262 the headquarters for the Interstellar Alliance on Minbar were finished and it was time for Sheridan and Delenn to leave B5. While en route to Minbar there was an accident on their whitestar, and Sheridan and a Ranger were locked into a room full of poisonous gas from the leaking coolant system. Lennier witnessed the men's plight, but in a bout of jealosy he left them to die. When Sheridan managed to save both himself and the Ranger, Lennier, suffering from pangs of guilt, ran away, but promised Delenn that one day he would come back.

When arriving to Minbar Sheridan and Delenn were surprised to find Londo waiting for them there. The emperor was acting amicably and even gave a gift, an ancient urn, to their unborn son. What the unsuspecting first couple of the Alliance didn't know was that the innocent-looking gift contained a 'keeper' that would take over the child on his sixteenth birthday.

Years went by for Delenn in more or less peaceful existence. She was now the vice-president of the Interstellar Alliance, but she made sure she gave enough time to her son David as well. In 2277 all her nightmares came true when the Drakh keeper in the urn took over David and the boy ran off to Centauri Prime. Both Delenn and Sheridan followed him there, even though they knew that they were walking straight into a trap. The insane Prime Minister Durla and the powerless Emperor Londo, both controlled by the Drakh, had the presidential couple imprisoned. Sheridan and Delenn would have been executed had it not been for Londo who sacrificed his life to ensure their, and David's, escape. When the Drakh were defeated some time later, David was freed from his keeper.

Delenn continued her work as Ranger One and the vice-president of the Alliance until 2279 when Sheridan retired and Delenn was elected as his follower. With the heavy burden of presidency now in her hands, Delenn in return delegated the job of the Ranger One to Sheridan.

Despite of all the tragedies in their lives Sheridan and Delenn were happily married for almost twenty years, right until he died in 2281. Delenn outlived her husband by over eighty years, being still alive in 2362, though over 140 years old.