Brennan was born in Swampscott, Massachusetts to Irish emigrants, and studied engineering in Cambridge, Massachusetts.

He is considered by many to have been one of the finest character actors in motion picture history. While the roles Brennan played were extremely diverse, he is often remembered for his portrayals in movie Westerns. He was the first actor to win three Academy Awards. He remains the only person to have won three Academy Awards for Best Supporting Actor.

While in school, he became interested in acting, and began to perform in vaudeville. After serving in World War I (where, according to legend, his vocal cords were damaged by poison gas), he moved to Guatemala and raised pineapples, before settling in Los Angeles, California.

The first years of his career saw Brennan working as an extra, with the occasional bit part in addition to being a stunt man. In the early 1930s he began receiving more substantial roles as his talent was recognized. This culminated with his receiving the very first Academy Award for Best Supporting Actor in 1937 for his role in Come and Get It.

Throughout his career, Brennan was frequently called upon to play characters considerably older than he was in real life. He was particularly skilled in playing the hero's sidekick or as the "grumpy old man" in a picture.

Other films included Red River, Rio Bravo, My Darling Clementine, Meet John Doe, The Pride of the Yankees, To Have and Have Not, Bad Day at Black Rock and How the West Was Won.

In the 1950s, he starred in the television series The Real McCoys, and appeared in several other movies and television programs, usually as an eccentric "old-timer" or "prospector". He also made a few recordings, the most popular being "Old Rivers" in 1962.

In 1967, he starred in the television series The Guns of Will Sonnett, where he played a father in search of his gunfighter son, James, with his grandson, Jeff, played by Dack Rambo.

Brennan was politically conservative, and supported American Independent Party candidate (and former governor of Alabama) George Wallace over Republican Party candidate (and, at the time, former Vice President) Richard Nixon during the 1968 Presidential campaign because he felt Nixon was too liberal.

For his contribution to the television industry, Walter Brennan has a star on the Hollywood Walk of Fame at 6501 Hollywood Blvd. In 1970, he was inducted into the Western Performers Hall of Fame at the National Cowboy & Western Heritage Museum in Oklahoma City, Oklahoma.

On his death from emphysema, aged 80, on September 21, 1974 in Oxnard, California, Walter Brennan was interred in San Fernando Mission Cemetery in Los Angeles, California. Brennan's wife, Ruth, who lived to be 100, is buried next to him. She died in 1997.