For the primary universe counterpart, see Sean Finnegan.

In the mirror universe, Sean Finnegan (later known as Black Jack) attended Starfleet Academy with James T. Kirk and Simon Van Gelder during the mid 23rd century.

While he was an upperclassman, he took great pleasure in bullying Kirk. However, with Van Gelder's assistance, Kirk murdered Finnegan while still attending the Academy. Kirk later commented that "[e]ven death couldn't wipe that stupid grin" off Finnegan's face. (TOS - Mirror Universe short story: "The Greater Good")

According to Phillipa Georgiou, Finnegan survived the beating at the Academy, but just barely. He had brain damage, so her surgeons fixed him up, while removing any part they didn’t feel was important, like self-preservation and compassion. Not knowing his actual name, she nicknamed him Black Jack and made him her enforcer and executioner. The brain damage has turned him into a brutal serial killer, who delighted in beating people to death with his truncheon, occasionally even sticking it into their eye sockets. As emperor, Georgiou would often sic Black Jack on those she felt didn’t deserve a quick death. (DIS - novel Die Standing)

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