Stack was born in Los Angeles, California, but spent his early childhood growing up in Europe. He became fluent in French and Italian at an early age, but he did not learn English until returning to Los Angeles. Raised by his mother, Mary Elizabeth (née Wood), Stack's parents divorced when Stack was a year old, and his father, James Langford Stack, a wealthy advertising agency owner, died when Stack was nine. Stack always spoke of his mother with the greatest respect and love. When he wrote his autobiography, Straight Shooting, he included a picture of him and his mother. He captioned it, "Me and my best girl." Stack's grandfather was an opera singer from Illinois named Charles Wood, who went by the name "Modini."
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.
Stack was a regular columnist for Gun World magazine.
