Rollin Hand, an actor, a magician, and a master of disguises and voices who billed himself as "The Man Of A Million Faces" and "The World's Greatest Impersonator," was a member of the Impossible Missions Force in the 1960s, and he was called upon to help accomplish missions which former U.S. Army Lt. Colonel Dr. Daniel David Briggs, Ph.D. and James "Jim" Phelps had accepted.
In the world of grand-scale confidence games, which method the IMF used most commonly, Rollin was both a "grifter" and a "roper," who first, as a grifter, sought and usually obtained the confidences of the mission targets and then, as a roper, directed them into the "big-store" settings that undid them.
Rollin's portrait for I.M.F selection.
Hand was portrayed by Martin Landau, who was already somewhat prominent before accepting the role. Notably, he had rejected the opportunity to portray Spock on Star Trek, which Leonard Nimoy later filled. Landau and his then wife, Barbara Bain, left the show after Season 3, and Nimoy took his place as The Great Paris, who filled basically the same role as Rollin.
It is unknown if Hand retired, was transferred to Government work elsewhere, or remained with the IMF accomplishing missions with some other IMF agents. Since Hand was a magician, an actor, and a master of disguise, that was likely how the IMF discovered him.