Robert Stack was born Charles Langford Modini Stack in Los Angeles, California, but his first name (selected by his mother) was changed to Robert by his father. He spent his early childhood growing up in Europe where he became fluent in French and Italian.
His family was one of the founding clans of California. His father, James Langford Stack, a wealthy advertising agency owner, died when Stack was nine. By the time he was twenty, Stack had achieved minor fame as a sportsman. He was an avid polo player and shooter. He and his brother won the International Outboard Motor Championships, in Venice, Italy; and, at age 16, he became a member of the All-American Skeet Team. He set two world records in skeet shooting and became National Champion. In 1971, he was inducted into the National Skeet Shooting Hall of Fame.