He is best known for his role as recurring villain Gul Dukat in the TV series Star Trek: Deep Space Nine.

Some of his theatrical roles include Iago in Othello, Rodolpho in Arthur Miller's A View From the Bridge and Lucky in Samuel Beckett's Waiting for Godot.

He has appeared, mostly as villains, in shows such as The Doctors, Kojak, Gunsmoke, Baretta, The Six Million Dollar Man, The Bionic Woman, Starsky & Hutch, Knight Rider, Quincy, The Greatest American Hero, The Incredible Hulk, Quantum Leap, Family Guy, Walker, Texas Ranger, Wonder Woman, The Rockford Files, Hill Street Blues, Barnaby Jones, and The A-Team.

In 1993, Alaimo began playing Cardassian character Gul Dukat in Star Trek: Deep Space Nine.

During a Q&A panel at the Star Trek Las Vegas Convention in 2015, Alaimo revealed he was still willing to work as an actor but was out of touch with the industry and had no agency representing him at that time.