At the age of seventeen, Coleman disobeyed his parents' wishes and joined Starfleet with his best friend, Robert Smith. While attending Starfleet Academy, he also became close friends with Jacob Sinclair. All three graduated together in 2330.
When the Merrimac responded to an attack at Starbase 53, Coleman met Admiral Admiral John Harriman. Harriman saw great potential in Coleman and took the young officer under his wing, eventually recruiting him into Starfleet Intelligence.
As an intelligence operative, Lieutenant Coleman was frequently partnered with Jacob Sinclair, and the two forged a close partnership. In 2338, Coleman was captured by the Cardassians on an undercover mission, one of the first encounters between Starfleet Intelligence and the Obsidian Order. Sinclair defied orders and went back to extract him, and Coleman later confessed, "Until Jake showed up, I thought I was going to die in that rathole." Some of Coleman's later missions also involved James Leyton, Alynna Nechayev, and Elias Vaughn.
In early 2341, Coleman and Sinclair were assigned to evaluate classified work being done at Enclave J-12 on Titan. After their initial investigation, they came to believe something was going on at the Enclave, other than what they'd been told. Coleman presented their findings to Admiral Harriman, who ordered an investigation "at a higher level" and reassigned the partners to another case. Though Mark advised him against it, Sinclair continued to pursue the matter discreetly. In June, he brought his conclusions to Coleman, and the two arranged a rendezvous at Titan with Vaughn, Nechayev and Thomas Allan. Before the rendezvous, Mark relayed some of the data to Harriman, requesting logistical support. When Coleman and the others arrived at the coordinates, they found a recovery operation in progress and learned that Sinclair and his wife had been involved in a "shuttle accident" which claimed their lives, and that of their unborn son. The incident was classified, and all of Coleman's attempts to learn the truth were blocked.
In 2358, Coleman conducted an Internal Affairs investigation of the loss of the USS Pegasus. During the course of his investigation aboard the USS Beachmont, he met young Timothy Sinclair, whom he had believed dead in the shuttle crash that killed his parents seventeen years earlier. Coleman later sponsored Sinclair for Starfleet Academy, overruling the objections of Enclave J-12.
In the aftermath of the Federation Civil War, Coleman was appointed Director of Starfleet Intelligence over Nechayev's objections. Over the five-year period of reconstruction, Coleman and Nechayev often found themselves at odds over ideology and tactics, especially after Coleman allied himself with Fleet Admiral James Cannon and his plans for Project Restoration.
When the Federation Council approved the plans to send a vessel to establish order in the Demilitarized Zone, Coleman traveled to Colorado to convince Timothy Sinclair to return to Starfleet and assume command of the USS Pendragon. (PDN: "True North")