Born in Glen Dale, West Virginia, Brett was the youngest of four sons of a sports-minded family which included Ken, the second oldest, a major league pitcher who pitched in the 1967 World Series at age 19. Brothers John (eldest) and Bobby had brief careers in the minor leagues. Although his three older brothers were born in Brooklyn, George was born in the northern panhandle of West Virginia. Jack and Ethel Brett then moved the family to the Midwest and three years later to El Segundo, California, a suburb of Los Angeles, just south of Los Angeles International Airport.

Following his playing career, Brett became a vice president of the Royals and has worked as a part-time coach, as a special instructor in spring training, as an interim batting coach, and as a minor league instructor dispatched to help prospects develop. He also runs a baseball equipment and foam-hand company, Brett Bros., with Bobby and, until his death, Ken Brett. He has also lent his name to a restaurant that failed on the Country Club Plaza.  In 1992, Brett married the former Leslie Davenport, and they reside in the Kansas City suburb of Mission Hills, Kansas. The couple has three children: Jackson (named after George's father), Dylan (named after Bob Dylan), and Robin (named after fellow Hall of Famer Robin Yount of the Milwaukee Brewers).  Brett has continued to raise money for amyotrophic lateral sclerosis (ALS), commonly known as Lou Gehrig's disease. Brett started to raise money for the Keith Worthington Chapter during his playing career in the mid-1980s.  He and his dog Charlie appeared in a PETA ad campaign, encouraging people not to leave their canine companions in the car during hot weather. He also threw out the ceremonial first pitch to Mike Napoli at the 2012 Major League Baseball All-Star Game.  On May 30, 2013, the Royals announced that Brett and Pedro Grifol would serve as batting coaches for the organization. On July 25, 2013 (the day following the 30th anniversary of the "pine tar incident"), the Royals announced that Brett would serve as Vice President, Baseball Operations.

What else did Brett do with PETA?