Born in 1894 in Lynn, Massachusetts, Walter Brennan was the son of Irish immigrants. He studied engineering in Cambridge, Massachusetts, and became interested in acting in school plays. During his summer breaks, he worked as a bank clerk and lumberjack.
Before entering the military in 1917, he toured in small musical comedy companies. After serving in World War I, he lived in Guatemala, where he raised pineapples. He then migrated to Los Angeles, where he speculated in real estate and, after losing all his investments, began working as a film extra and then as a stuntman in 1923.
Eventually, Brennan began to get speaking parts and in 1936 won an Academy Award for Best Supporting Actor for the movie Come and Get It. He won two more Academy Awards and was nominated a fourth time. In the 1950s, he starred in the television hit series The Real McCoys, as well as appearing in numerous other television shows and movies throughout his acting career.