After outscoring his fellow soldiers on an officer's exam, he became Captain in the Royal Canadian Artillery. While leading his men into battle on D-Day, Doohan was wounded in the leg and hand, and eventually lost a finger. For the remainder of the war, he became a pilot observer, and received the dubious distinction of being called the "craziest pilot in the Canadian Air Forces."

After returning home to Canada, Doohan performed a few scenes for the local radio station, and was awarded a two-year scholarship to the Neighborhood Playhouse in New York City. Beginning in 1946, he trained at the Playhouse with Sanford Meisner, alongside such future stars as Leslie Nielsen, Tony Randall and Jackie Gleason.

In the ensuing eight years, Doohan shuttled between New York and Canada where he worked on 4000 radio programs, 400 live and taped variety and dramatic television shows, several films and plays. Though he became known as Canada's busiest actor, he eventually found himself following other fellow actors in the pilgrimage to Hollywood. There, his versatility and talent as a dialectician helped him earn parts in more than 100 motion pictures and television series, including The Twilight Zone, Outer Limits, Fantasy Island, Loaded Weapon and Double Trouble. He has also appeared in the first seven Star Trek motion pictures.

In the years since the final episode of Star Trek, Doohan has pursued a speaking career which has taken him to more than 250 colleges throughout the U.S. and Canada. In addition, he makes appearances at numerous Star Trek conventions.

Doohan lives in Washington, with his wife Wende and his sons, Eric and Thomas.