"Mineo was baffled by his sudden loss of popularity, later saying, "One minute it seemed I had more movie offers than I could handle; the next, no one wanted me".
Mineo met actress Jill Haworth at the set of the film Exodus, where they played young lovers... Mineo and Haworth were together on-and-off for many years, even getting engaged to be married at one point, though she canceled the engagement when she became aware of an affair Sal was having with Bobby Sherman. They did remain very close friends until Mineo's death.
Mineo expressed disapproval of Haworth's brief relationship with the much older television producer Aaron Spelling. Haworth was 20 and Spelling was 42. One night when Mineo found Haworth and Spelling at a private Beverly Hills nightclub, he walked up and punched Spelling in the face, yelling, "Do you know how old she is? What are you doing with her at your age?"
In a 1972 interview with Boze Hadleigh, Mineo discussed his bisexuality. At the time of his death, he was in a six-year relationship and was living with male actor Courtney Burr III.
Michael G. Michaud wrote a biography of Mineo with the majority of information coming from Haworth and Burr. In his book, Michaud claimed that Mineo had sexual relations with then teen idol Bobby Sherman. He claimed Mineo never had any sexual relations with either James Dean or Don Johnson. Johnson and Mineo had been roommates for a time and became friends. Mineo was also close friends with David Cassidy, another teen idol.
By 1976, Mineo's career had begun to turn around. While playing the role of a bisexual burglar in a series of stage performances of the comedy P.S. Your Cat Is Dead in San Francisco, Mineo received substantial publicity from many positive reviews and he moved to Los Angeles along with the play.
Mineo was arriving home after a rehearsal on February 12, 1976, when he was stabbed to death in the alley behind his apartment building near the Sunset Strip in West Hollywood, California. Mineo was stabbed just once, not repeatedly as first reported, but the knife blade struck his heart, leading to immediate and fatal internal bleeding. His remains were interred in the Gate of Heaven Cemetery in Hawthorne, New York.
In March 1979, pizza deliveryman Lionel Ray Williams was sentenced to 57 years in prison for killing Mineo and for ten robberies in the same area. Although considerable confusion existed as to what witnesses had seen in the darkness the night Mineo was murdered, Williams claimed to have had no idea who Mineo was. Corrections officers later said they had overheard Williams admitting to the stabbing. Williams was defended by Mort Herbert."