Benjamin Sisko was a Terran privateer who worked his way out of the Klingon-Cardassian Alliance's slave mines and earned himself a position serving the Intendant of Bajor, Kira Nerys. Sisko was in addition one of Kira's lovers.
Sisko commanded a small raiding ship crewed by other Terrans, charged with collecting "duties" from passing ships for Kira Nerys. Although Sisko resented his position under the Intendant, he had no desire to resist the Alliance's rule. However, when he met the Kira Nerys from the mirror universe in 2370, Sisko was lured into starting the Terran Rebellion. He helped Kira and Bashir, another mirror universe troublemaker, escape from Terok Nor. From that day forth, Sisko was a terrorist bent on the destruction of the Alliance. (DS9: "Crossover")
From 2371, little is known of Sisko's movements. Reports are that he is either dead or on a mission to enlist aid from the Romulans.
Although Sisko was a great tactician, he was a stubborn, abrasive man who didn't get along with anyone very well. He was married to Jennifer Sisko, a privileged Terran scientist, but hardly got along with her at all, and as of 2371 hadn't seen her in over five years. According to Smiley, they never should have been married in the first place. Sisko was the lover of Jadzia Dax, whom he recruited to join the rebellion.