Leibman was a member of the Compass Players in the late 1950s, and was admitted to the Actors Studio shortly thereafter.
Leibman won an Emmy Award, Outstanding Lead Actor In A Drama Series, in 1979 for his convict-turned-lawyer character in Kaz (1978–79), a series which he also created and co-wrote.
He co-starred with second wife Jessica Walter in Tartuffe at the Los Angeles Theatre Center in 1986, and they co-starred again in Neil Simon's play Rumors in 1988 on Broadway. They also appeared together as husband and wife in the film Dummy (2003) and in the TV series Law & Order in the episode "House Counsel" in 1995.
Leibman received a 1993 Tony Award for his performance as Roy Cohn in the Pulitzer Prize-winning play Angels in America.
In 2013, Leibman began appearing as a recurring character on the TV series Archer as Ron Cadillac. He stars as the husband to Malory Archer, voiced by his real life wife Jessica Walter.