What was the name of the actor who played the Man in Black?

The actor who played the Man in Black in Lost was Titus Welliver.

Welliver portrayed the character in his true human-looking form, particularly after the Man in Black's identity was revealed during the fifth-season finale, "The Incident." Before that revelation, the character was primarily seen as the terrifying Smoke Monster, a cloud of black smoke that hunted and judged people on the island.

The Man in Black was originally a human living on the island and the twin brother of Jacob. After Jacob threw him into the island's mysterious source of light, black smoke emerged, suggesting that the entity became the Smoke Monster. The Monster could scan memories and assume the appearances of dead people.

That ability explains a frequent casting mix-up. Terry O'Quinn played the Man in Black while he was impersonating John Locke, while John Terry, Adetokumboh M'Cormack, and other actors portrayed forms connected to dead characters. Titus Welliver remains the canonical actor for the Man in Black's real form.