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 one and his father, James Langford Stack, a wealthy advertising agency owner, died when Stack was nine. http://louis0j0sheehan0esquire.blogspot.comStack 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.http://louis0j0sheehan0esquire.blogspot.com

By the time he reached 20 Stack achieved minor fame as a sportsman. Robert Stack was an avid polo player. He and his brother won the International Outboard Motor Championships in Venice, and at the age of 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.http://louis0j0sheehan0esquire.blogspot.com

Stack took drama courses at the University of Southern California. His deep voice and good looks attracted producers in Hollywood. When Stack visited the set of Universal Studios at age 20, producer Joe Pasternak offered him an opportunity to enter the business. Recalled Stack, "He said 'How'd you like to be in pictures? We'll make a test with Helen Parrish, a little love scene.' Helen Parrish was a beautiful girl. 'Gee, that sounds keen,' I told him. I got the part." Stack's first film, which teamed him with Deanna Durbin, was First Love in 1939. He was the first actor to give Durbin an on-screen kiss. As hard as it is to believe today, this film was considered controversial at the time.

Stack won acclaim for his next role, 1940's The Mortal Storm. He played a young man who joins the Nazi party. This film was one of the first to speak out against Hitler. As a youth, Stack admitted that he had a crush on Carole Lombard and in 1942 he appeared with her in To Be or Not To Be. He admitted he was terrified going into this role. He credits Lombard with giving him many tips on acting and with being his mentor. Lombard was killed in a plane crash shortly before the film was released.

During World War II, Stack served as gunnery instructor in the United States Navy. He continued his movie career and appeared in such films as Fighter Squadron (1948), A Date with Judy (1948) and Bwana Devil (1952). In 1954, Stack was given his most important movie role. He appeared opposite John Wayne in The High and the Mighty. Stack played the pilot of an airliner who comes apart under stress after the airliner encounters engine trouble.

In 1957, Stack was nominated for an Academy Award for Best Supporting Actor for Written on the Wind. He starred in more than 40 films. Known for his steadfast, humorless demeanor, he made fun of his own persona in comedies such as 1941 (1979), Airplane! (1980), Caddyshack II (1988), and BASEketball (1998). He also provided the voice for the character Ultra Magnus in Transformers: The Movie (1986).

Stack depicted the crimefighting Eliot Ness in the television drama The Untouchables from 1959 to 1963. The show portrayed the ongoing battle between gangsters and federal agents in a Prohibition-era Chicago. The show brought Stack a best actor Emmy Award in 1960. The Untouchables was a "realistic" cop show, in the tradition of Dragnet. Stack also starred in three other series, rotating the lead with Tony Franciosa and Gene Barry in the lavish The Name of the Game (1968-1971), Most Wanted, (1976) and Strike Force (1981). Interestingly, in The Name of the Game, he played a former federal agent turned true-crime journalist, evoking memories of his role as Ness. In both Most Wanted and Strike Force he played a tough, incorruptible police captain commanding an elite squad of special investigators, also evoking the Ness role. Eventually, he would reprise the role in a 1992 TV movie, The Return of Eliot Ness.

He began hosting Unsolved Mysteries in 1988, where his serious, ominous voice and stoic facial expressions lent an authentic gravitas to the program's dark subject matter. Reportedly, he had an enormous interest in the unexplained—psychic phenomena, ghosts and the like—because he himself had had an unusual experience of this nature. However, he also said that he valued the storytellers above the stories themselves and did not necessarily believe every case of this nature that he presented. He thought very highly of the interactive nature of the show, saying that it created a "symbiotic" relationship between viewer and program, and that the hotline was a great crime-solving tool. Unsolved Mysteries aired from 1988 to 2002, first on NBC from 1987 as specials (Stack did not host all the specials), then as a series from 1988-97, then on CBS (1997-99) and finally on Lifetime in 2001-02. Stack served as the show's host during its entire series run.

Stack had undergone radiation therapy for prostate cancer in October 2002. He died of heart failure at his home in Los Angeles On May 14, 2003.

Actress Rosemarie Bowe was married to Stack from 1956 until his death in 2003.http://louis0j0sheehan0esquire.blogspot.com

Stack was the great-uncle of actor Taran Killam. He is interred in the Westwood Village Memorial Park Cemetery in Westwood, California.