He is perhaps most famous for his leading role as Hawkeye Pierce in the hit television series M*A*S*H. During the course of the series' eleven year run on CBS, Alda was nominated for 21 Emmys. He won five awards, and became the first person to win the award as an actor, writer and director.

He has also won the Writer's Guild Award twice, the Director's Guild Award three times, six Golden Globes from the Hollywood Foreign Press Association as Best Comedy Actor, and seven People's Choice Awards.

After M*A*S*H

Alda's prominence in the enormously successful M*A*S*H gave him a platform to speak out on political topics, and he has been a strong and vocal supporter of women's rights. In 1976, the Boston Globe dubbed him "the quintessential Honorary Woman: a feminist icon" for his activism on behalf of the Equal Rights Amendment. As a liberal activist, he has been a target for some political and social conservatives.

Alan Alda has also played Nobel Prize-winning physicist Richard Feynman in the play QED, which has only one other character. Although Peter Parnell wrote the play, Alda both produced and inspired it. Alda has also appeared frequently in the films of Woody Allen, and he has been a guest star five times on ER, playing Dr. Kerry Weaver's mentor, Gabriel Lawrence. During the later episodes, it was revealed that Dr. Lawrence was suffering from the early stages of Alzheimer's or dementia. Alda also had a co-starring role as Dr. Robert Gallo in the controversial 1993 TV movie And The Band Played On.

During M*A*S*H's run and continuing through the 1980s, Alda embarked on a successful career as a writer and director, with the ensemble dramedy The Four Seasons being perhaps his most notable hit. 1990s Betsy's Wedding is his last directing credit to date. After M*A*S*H Alda took on a series of roles that either parodied or directly contradicted his "nice guy" image. His role as a pompous celebrity comedian in Crimes and Misdemeanors was widely seen as a self-parody, although Alda denied this.

In 1995 he briefly considered running for the United States Senate in New Jersey. About this time, he starred as the President in Michael Moore's Canadian Bacon. Beginning in 2004, Alda was a regular cast member on the NBC program The West Wing, portraying Republican U.S. Senator and presidential hopeful Arnold Vinick, until the show's conclusion in May 2006. He made his premiere in the sixth season's eighth episode, "In The Room," and was added to the opening credits with the thirteenth episode, "King Corn." In August 2006, Alda won an Emmy for his portrayal of Arnold Vinick in the final season of The West Wing.

Alda has done extensive charity work. He helped narrate a 2005 St. Jude's Children's Hospital produced one-hour special TV show Fighting for Life. He is friends with Marlo Thomas, who is active in fund raising for the hospital her father founded. The special featured Ben Bowen as one of six patients being treated for childhood cancer at Saint Jude.

Alda also wrote several of the stories and poems that appeared in Marlo Thomas's Free to Be... You and Me television show.

Throughout his career, he has been nominated for the Emmy Award 31 times and the Tony Award twice, and has won seven People's Choice Awards, six Golden Globe awards, and three Directors Guild of America awards. However, it was not until 2004, after a long acting career, that Alda received his first nomination for an Academy Award for his supporting role as Senator Ralph Owen Brewster in Martin Scorsese's film The Aviator.

In the spring of 2005, Alda starred as Shelly Levene in the Tony Award-winning Broadway revival of David Mamet's Glengarry Glen Ross, for which he received a Tony Award nomination for Best Featured Actor in a Play.

It has become quite common for Alda in his later roles to have some reference to his early work in M*A*S*H. In a line on ER, his character mentions that he uses a surgical technique that is "an old army trick." Alda's West Wing character has also made at least one reference to Korea when he said, "I could take these people to the DMZ and it still wouldn't take their minds off ethanol and abortion."

In 2005, Alda published his first round of memoirs, Never Have Your Dog Stuffed: and Other Things I've Learned. Among other stories, he recalls his intestines becoming strangulated while on location in Chile for his PBS show Scientific American Frontiers, during which he mildly surprised a young doctor with his understanding of medical procedures, which he learned from M*A*S*H. He also talks about his mother's battle with schizophrenia. The title comes from an incident in his childhood, when Alda was distraught about his dog dying and his well-meaning father had the animal stuffed. Alda was horrified by the results, and took from this that sometimes we have to accept things as they are, rather than desperately and fruitlessly trying to change them.

In 2006, Alda contributed his voice to a part in the audio book of Max Brooks' World War Z. In this book, he voiced Arthur Sinclair Jr., the director of the United States Government's fictional "Department of Strategic Resources (DeStRes)".