Galecki was born in Bree, Belgium, of Polish, Irish, and Italian descent. His mother, Mary Lou, was a mortgage consultant, and his father, Richard Galecki, was a member of the U.S. Air Force stationed in Belgium and also worked as a rehabilitation teacher. Galecki moved to Oak Park, Illinois, with his family when he was three years old.
During an interview with New Zealand radio station 'ZM', Galecki jokingly recalled his childhood relationship with his mother. As a child, he was well known for making up long, epic stories and tales. In such situations his mother used to make him play the "quiet game" where he had to see how long he could go without talking. He also recalled that despite being a loving mother, she was also very tough. One phrase she would lovingly use was, "I love you, now get out."