Born in 2226, Marcus took over his family business, a mining company, after his parents' death. He had a younger brother, William, but the two didn't get along well especially after William left their homeplanet, Arisia, to explore the galaxy. During his travels, William joined the Rangers (who in Marcus' opinion were nothing but a highly suspicious cult). After his training William returned to Arisia and was reunited with his brother. Unfortunately the Shadows chose just that time to attack the planet and killed not only William but also a woman Marcus was very fond of. Marcus, the only survivor, was left with nothing and to honour his brother's memory he decided to join the Rangers.

After his training Marcus ended up on Babylon 5. He first came there in early 2260 to ask Captain Sheridan and Delenn to rescue a group of Rangers from the Shadows, but he soon made friends on the station and remained there, fighting in both the Shadow War and Earth's Civil War. He helped Commander Ivanova find First Ones, acted as a second-in-command for both her and Sheridan on whitestar during various battles and even worked as Doctor Franklin's bodyguard when the two of them were sent to Mars to meet with the resistance leaders.

For a very long time Marcus was in love with Ivanova, but he couldn't gather up enough courage to tell her about his feelings until it was too late. When Ivanova was fatally wounded during the Civil War in 2261, he stayed by her side wanting to do everything in his power to help her. After finding out about the alien healing device, a machine that transferred life-energy from one person to another, he stole a whitestar and left the battlefield. On B5 he broke into MedLab and gave all his life-energy to Ivanova. She recovered completely, but he died. After her recovery Ivanova had Marcus cryogenically frozen in the hope that he could one day be resurrected.