First commanding officer of Babylon 5, assigned there in 2257. Previously, he had been a fighter pilot, and fought at the Battle of the Line. He was unable to remember what had happened to him at the battle until later.

In 2259, Sinclair was reassigned as Earth's ambassador to Minbar. There, he took over leadership of the Anla'shok. In 2260, he assisted in securing Babylon 4 and traveled back in time a thousand years with it, to become Valen.