Brimley was born in Salt Lake City, Utah where his father worked as a real estate broker.
Diagnosed with diabetes in 1979, Brimley began working to raise awareness of the disease. The American Diabetes Association (ADA) honored Brimley in 2008 with an award to recognize his lifetime of service in this venerable cause. Brimley has been active visiting Veterans Administration hospitals and communities to advise patients on how to manage their diseases. The ADA presented the award to the actor at the Port St. Lucie headquarters of Liberty Medical on December 19, 2008.
Brimley is an activist, paying from his own funds for ads to have Utah allow horse-race gambling, and he was actively opposed to the banning of cockfighting in New Mexico. Brimley enjoys playing poker and has played in the World Series of Poker Main Event. Brimley lent his support to John McCain in the 2008 U.S. presidential election. In the days leading up to his selection for vice president, McCain stated that he would pick Brimley: "He's a former Marine and great guy and he's older than I am, so that might work."