Galecki was born in Bree, Limburg, Belgium to American parents of Polish, Irish, and Italian descent. His mother, Mary Louise (Mary Lou) Noon, was a mortgage consultant, and his father, Richard Galecki, was a member of the U.S. Air Force stationed in Belgium and worked as a rehabilitation teacher. Galecki is the eldest of three children with a sister, Allison, and a brother, Nick, whom he describes as a "mechanical genius" in the automotive industry. Galecki grew up in Oak Park, Illinois. He dropped out of school after 8th grade, attending high school for only one day.
During an interview with New Zealand radio station ZM, Galecki recalled his childhood relationship with his mother. As a child, he was well known for making up long 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."
