Jeff Sinclair was born on Mars on May 3rd 2218. He had a catholic upbringing and even spent three years of his youth in jesuit school. There had been fighter pilots the Sinclair family since World War II and following the footsteps of his father who had served in the Dilgar War, Jeff too joined Earth Force in 2237. He became a fighter pilot in 2240 and was promoted a squadron leader less than a year later. It was around the same time that he met the true love of his life, Catherine Sakai. The two lived together for a year before breaking up, a pattern which they would repeat several times during the next twenty years.

When the Earth-Minbari War broke out, Sinclair too joined the fight. During the Battle of the Line he lost his entire squadron in just a matter of minutes. His own starfury was also severely damaged and he decided to ram into a minbari ship to take at least a few of the enemy with him. He didn't succeed, though, as Delenn had him captured before his starfury hit the ship. She had given the order for his capture to find out about Earth's defencies, but in the end his interrogation revealed that the Minbari were reborn as humans and that Sinclair himself had the soul of none other than Valen himself. Shocked by this revelation the Minbari surrendered immediately and returned Sinclair (though not before removing all the memories he had of his visit to the Minbari ship). Although Sinclair was rewarded with a Silver Star for valour during the Battle of the Line, he soon realised that he had fallen off the promotions carousel. There was a 24-hour long hole in his mind and even though there were perfectly reasonable explanations for his disappearance, some people suspected that there was some connection between the missing hours and the Minbari surrender. It was only years later that Sinclair learned the truth.

In 2256 Sinclair, now a Commander in rank, became the commander and military governor of Babylon 5. He had not been the first choice for the position; in fact, he had been nowhere near the top of the list of candidates. He had only been chosen because of pressure from the Minbari (who were funding the Babylon Project). Unaware of being the unpopular choice, Sinclair took the job and immediately used his new authority to hire his friends Laurel Takashima and Michael Garibaldi as his second-in-command and security chief, respectively. He had hardly settled in when he was already being framed for the assasination attempt on Ambassador Kosh. Sinclair was eventually cleared of charges, but Takashima, Dr. Benjamin Kyle (the station's chief medical officer) and the resident telepath Lyta Alexander were called back to Earth for having opened Kosh's encounter suit without authorization and against the will of the Vorlon himself.

During the next year Sinclair too bended a few rules by transferring money from the defence budget to give the dockers a raise. His actions were investigated, but he managed to keep his reputation clean. In 2258 he also resumed his relationship with Sakai and this time the two decided to stay together.

Everything changed just minutes into the new yer 2259 as Earth Alliance President Louis Santiago was killed and Vice-President Clark assumed power. One of Clark's first actions as president was to re-assign Sinclair as Earth's ambassador to Minbar and to turn over the command of B5 to Captain Sheridan. History seemed to repeat itself as Sinclair had hardly settled in his new post before he was again accused of a murder attempt. Still, he soon earned the trust of the Minbari and was asked to take command of the Rangers. There was some objection to his appointment, but eventually he proved his skills. Sakai too had followed him to Minbar and joined the Ranger training program to be near him. Unfortunately during a mission her ship strayed into a time distortion in space and she was presumed dead.

In 2260 Sinclair received a sealed letter which he himself had written over 900 years earlier and which had then been waiting in a temple for this very day. Following the instructions in the letter he returned to B5 from where he, Sheridan, Delenn and a few others continued to Sector 14. Once there they took the White Star through a time distortion not unlike the one that had been the fate of Sakai, and travelled six years back in time to 'steal' Babylon 4. Sinclair and Zathras then took the station another thousand years into the past to help the Minbari in the previous Shadow War. During their journey through time he used the triluminary Zathras had brought along and transformed himself into a Minbari. He then became not only a part of the Minbari belief of the One - he was The One Who Was - but also their most revered religious leader Valen. In his lifetime he created the Grey Council, founded the Rangers and wrote the predictions which would guide the Minbari society through thousand years of peace.

After the Shadow War Valen retired from the public. He married, and it is quite possible that his wife was no other than Catherine Sakai, who through the time distortion had ended up in the same era as he. After his death his children had to flee Minbar in fear of persecution, but eventually they returned to their homeworld and introduced human DNA into the Minbari population.