Jeffrey David Sinclair was born on May 3, 2218 on Mars Colony. In an early episode of season one, Sinclair states that his family had been pilots "ever since the Battle of Britain" and Sinclair's father was a fighter pilot for EarthForce who participated in the Battle of Balos, the last engagement of the Dilgar Invasion. Sinclair continued the tradition and became a fighter pilot. Sinclair enlisted in EarthForce in 2237. During his time at EarthForce Academy, he met Catherine Sakai, with whom he had a relationship. After a year of living together, the two of them broke up, but continued to see each other off and on through 2258, when they became engaged. Sakai went missing in late 2259, while on a mission for the Rangers. The season one episode "By Any Means Necessary" establishes that Sinclair received Jesuit education as a young man.
In 2240, Sinclair was promoted to fighter pilot, continuing a Sinclair family tradition that had extended as far back as the Battle of Britain. Less than a year later, Sinclair was promoted to squadron leader; due to his rapid rise through the ranks, the rumor of the day was that Sinclair was on the fast track to making Admiral. As squadron leader, Sinclair fought at the Battle of the Line, the last major battle in the Earth-Minbari War. During the course of the battle, his squadron was destroyed by the Minbari, and his fighter was badly damaged. In a last act of defiance, Sinclair attempted to ram one of the Minbari cruisers. He failed in this when he was captured by another Minbari cruiser for interrogation by the Grey Council. The Council's Triluminary detected Valen's DNA in Sinclair, so they assumed, to their profound shock, that he possessed the soul of Valen, a hero of the Minbari who led them to victory 1,000 years ago against the Shadows. It was concluded by the Grey Council that Minbari souls were being born into human bodies. The discovery of what Sinclair possessed led the Minbari to surrender and return Sinclair to his fighter, the memory of his time aboard the Minbari cruiser blocked (though this block would not be permanent and would break down years later). Sinclair — and the Earth Alliance — believed that he had blacked out from the acceleration. When Babylon 5 was brought into operation in 2257, Sinclair was selected by the Minbari to command the newly constructed station. He was selected over many more senior officers, including Colonel Ari Ben Zayn, all of whom had been vetoed by the Minbari (they had stipulated that they should approve the choice of Station Commander, as they had shared the cost of construction).
In January 2259, Sinclair was reassigned as ambassador to Minbar, where he took command of the Rangers. He was succeeded at Babylon 5 by Captain John Sheridan. In 2260, Sinclair received a 900-year old letter from himself on Minbar, revealing that he was not the reincarnation of Valen, as the Grey Council believed, but in fact Valen himself. Armed with this knowledge, Sinclair took Babylon 4 back with him 1,000 years to aid the Minbari in their first war against the Shadows, and in so doing, fulfilled Minbari prophecy by becoming the One Who Was. It was here that Sinclair used the triluminary to transform himself into a Minbari, thus fulfilling the legend about Valen being "a Minbari not born of Minbari", also explaining why the triluminary responded so strongly to him during his interrogation by the Grey Council, as it had been programmed to respond to his DNA. It was later revealed that Delenn is a descendant from valen