Boyd was born William Millar in 1931 (some references say 1928). One of nine siblings, he attended Ballyclare High School. At the age of seven he became well known in Belfast for his contributions to the Ulster Radio's Children Hour. At the age of sixteen, Boyd quit school and joined the Ulster Group Theater. Boyd learned the behind the scenes tasks of the theater, and eventually worked his way up to character parts and leads, touring both Canada and the United States with stock companies.

By the time he was twenty, Boyd had a wide range of theater experience, but he longed for the big stage. In 1952 Boyd moved to London and worked in a cafeteria and busked outside a cinema in Leicester Square to get money as he was literally close to starvation. Boyd caught his first break as a doorman at the Odeon Theatre. The Leicester Square Cinema across the street recruited him to usher attendees during the British Academy Awards in the early 1950s. During the awards ceremony he was noticed by actor Sir Michael Redgrave, who used his connections to introduce Boyd to the Director of the Windsor Repertory Group.

In November 1956, for Twentieth Century Fox, Boyd traveled to the British West Indies as part of a large ensemble cast in Darryl Zanuck's racially provocative film Island in the Sun, based on the Alec Waugh novel. Boyd portrayed a young English aristocrat who becomes the lover of Joan Collins. Boyd would be loaned out to the J. Arthur Rank production of Seven Thunders (Beast of Marseilles), a World War II romance set in Nazi-occupied Marseilles. This movie was filmed on location in Marseilles and at Pinewood Studios in London in the spring of 1957 and featured Boyd in his most prominent starring film role yet.

Around the same time French Actress Brigitte Bardot was given the opportunity to cast her own leading man in her next movie after her success in Roger Vadim's And God Created Woman, and she chose Boyd. From August to October 1957, Bardot, Boyd and Alida Valli filmed the lusty romance The Night Heaven Fell, directed by Roger Vadim in Paris and in the region of Málaga, Spain, specifically the small, white-washed town of Mijas. Being in the Bardot spotlight added much to Boyd's film credit, in addition to bringing him notice in Hollywood.

Boyd was first married in 1958 to Italian-born MCA executive Mariella Di Sarzana during the filming of Ben-Hur. They separated after just three weeks. Concerning his short-lived marriage to Sarzana, Boyd would explain, "It was my fault. I'm an Irish so-and-so when I'm working. I hadn't been married a week when we both knew we had made a mistake. She is a nice girl but we were just not meant for each other. I suppose I wasn't ready for marriage. Maybe I was still too much of an adolescent." They officially divorced in early 1959.

Ben-Hur was released in December 1959 and made Boyd an international star overnight. His portrayal of the Roman tribune Messala brought in rave reviews. Press columnist Erskine Johnson wrote, "A brass hat and the armor of a Roman warrior in Ben-Hur does for Stephen Boyd what a tight dress does for Marilyn Monroe." Ruth Waterbury, in her Boyd feature in the Pittsburgh Post Gazette, would describe Boyd's character as "the dangerously masculine and quite magnificent Messala." Modern Screen magazine in 1960 stated that Boyd's ruthless Messala had "lost the chariot race but captured the sympathy and sex appeal of Ben-Hur."

Silver Screen Magazine in 1960 wrote this about Boyd:

Boyd flew to Rome in the summer of 1962 to act with Italian superstar Gina Lollobrigida in her long-time pet project Imperial Venus, a romantic epic about the many loves of Pauline Bonaparte, the sister of Napoleon. This film was the first film to be banned by the Motion Picture Association of America for male nudity. Boyd appeared in a humorous bedroom scene, naked, but covered by a sheet. The suggestion of nudity was too much for the censors and the movie was never released in the United States. Boyd returned to the States briefly after finishing Imperial Venus where he appeared for the second time on The Dinah Shore Chevy Show, which aired on November 11, 1962. For thus program Boyd was a last minute replacement for actor James Garner and joined Shore and entertainer Dean Martin for a few musical numbers.

Boyd also had a close relationship with Actress Dolores Hart. Hart describes what would be her only romance with a co-star in her autobiography The Ear of the Heart. Boyd eventually rejected her advances, but they remained close friends even after she turned to the cloistered life of a nun in 1963. He visited her in 1966 at the Abbey of Regina Laudis in Connecticut and remained in communication with her up until the early 1970s.

Throughout 1964 Boyd continued to make films in Europe, traveling to Yugoslavia to star as the villain Jamuga in the epic Genghis Khan. Boyd was the top billed and therefore the top paid star in the epic, and this apparently caused friction with up-and-coming star Omar Sharif. After completing Genghis Khan, Boyd trekked to Cairo, Egypt for a short stint in yet another epic, The Bible.

After all this globe-trotting, the world weary Boyd was very happy to return to the United States to start work on the Twentieth Century Fox science fiction adventure Fantastic Voyage, co-starring with soon-to-be icon Raquel Welch. This was filmed in the early part of 1965. In the summer of 1965, Boyd joined German star Elke Sommer and music legend Tony Bennett to film the Hollywood drama The Oscar, based on the eponymous Richard Sale novel. The movie was a popular success, but maligned by film critics. Boyd would make a 10-day visit to Iran in December 1965 to film his scenes for the United Nations film project The Poppy Is Also a Flower, written by James Bond creator Ian Fleming.

Journalist Florabel Muir described Boyd's appeal in a feature from 1966. "I would think it has to be his ruggedly masculine good looks. Strong, even craggy features, a wide sympathetic mouth, firm chin, athletic build, wavy dark brown hair, roving 185-lb. frame – all that plus a musical voice and the savoir faire of a much-traveled fellow – his films have taken him to many places in the world, and a rolling stone acquires a high Polish."

Next, Boyd starred in a spy thriller Assignment K with Swedish model/actress Camilla Sparv, which was filmed in Germany, Austria and London during February and March 1967. Boyd grew a full beard for his next role as the iconic Irish Playwright and critic George Bernard Shaw in the Off-Broadway play called The Bashful Genius written by Harold Callen. This was Boyd's first return to the stage since the mid-1950s, and the experience for Boyd was immensely rewarding on a personal level. He received excellent reviews for his nuanced performance of the multi-faceted Shaw. The play had a very brief run during the summer of 1967 in Denver, Philadelphia and Falmouth, Massachusetts.

In early 1968 Boyd was cast as the heavy opposite Sean Connery and Brigitte Bardot in the western adventure Shalako (film), based on the Louis L'Amour novel. Shalako was filmed in the early part of 1968 in Almería, Spain. After returning to the United States, Boyd took the role of the cruel slave master Nathan MacKay in the Southern "Slavesploitation" drama Slaves, also starring Ossie Davis and songstress Dionne Warwick. The film was loosely based on the famous Harriet Beecher Stowe novel Uncle Tom's Cabin. It was filmed during the summer of 1968 at the supposedly haunted Buena Vista plantation near Shreveport, Louisiana . The film was released during the volatile civil rights era and in May 1969 Boyd attended the premiere alongside Dionne Warwick in Baltimore, Maryland Closely following Slaves, Boyd starred in another story about racial tension, this time a World War II made-for-television drama called Carter's Army (or Black Brigade) which aired in August 1970, featuring a young Richard Pryor.

During this time, or earlier, is when Boyd began his interest in L. Ron Hubbard's Church of Scientology, which would make him one of the first Hollywood stars to be involved in it. Boyd had always expressed an interest in esoteric religions. In an interview in August 1969 with the Detroit Free Press, Boyd explained that Scientology had helped him through the filming of Slaves, and that to him Scientology was "a process used to make you capable of learning. Scientology is nothing. It means only what you want it to. It is not a church you go to pray, but a church that you go to learn. It is no good unless you apply it. It is the application." Boyd apparently had been elevated to a Scientology Status of OC 6, a position beneath that of Clear. Boyd would actually go on to star and narrate a Scientology recruiting film called Freedom in 1970. A copy of this film can be found at the Library of Congress, but it is not available online via any Scientology resource, which may indicate a falling out Boyd had with Scientology using his name for recruiting purposes. There is no documentation of his later involvement with Scientology after the early 1970s.

Boyd's last marriage took place in 1974 to Elizabeth Mills, a secretary at the British Arts Council, whom he had known since 1953. Elizabeth Mills followed Boyd to the United States in the late 1950s and was his personal assistant, friend and confidante for many years before marrying him in the mid- 1970s.

Boyd died of a massive heart attack on 2 June 1977 at the age of 45 while playing golf with his wife Elizabeth Mills at the Porter Valley Country Club in Northridge, California. "Stephen and Elizabeth were in a golf cart between the fifth and sixth tees when suddenly he said. 'I don't feel well,' and slumped over. Elizabeth dragged him out of the cart and gave him artificial respiration, but it was too late." He was in talks to play the role of the Regimental Sergeant Major in Euan Lloyd's The Wild Geese before his death. Boyd was interred in Oakwood Memorial Park Cemetery in Chatsworth, California. His wife Elizabeth Mills Boyd was interred with him at the time of her death in 2007.

Boyd had a deep and lasting friendship with Actress and French icon Brigitte Bardot. Boyd starred in two movies with Bardot — The Night Heaven Fell in 1958 and Shalako in 1968. During the filming of Shalako in Almeria, Spain, Bardot and Boyd's close relationship and open affection for each other sparked numerous rumors of a possible affair. It even caused Brigitte's husband at the time, Gunter Sachs, to ask for a divorce. In Bardot's autobiography, she described the events and states that Boyd "was never her lover, but a tender and attentive friend." In an interview with Photoplay Film in 1968, Boyd said, "Bardot is always Bardot. She's marvelous. She's an enormous star and she's a unique, marvelous woman. I adore her." Even though both actors denied the affair, the press was "convinced there was a romance afoot, that Brigitte and Boyd openly displayed their affection for each other, but that publication of the report on their romance cooled it."

Stephen Boyd’s most passionate affair seems to have been with beautiful Austrian Actress Marisa Mell. They met while filming the movie Marta in 1970. Boyd initially dodged Marisa Mell's amorous advances, but during the second film they made together, The Great Swindle, the two became inseparable lovers. They married in a gypsy camp on the outskirts of Madrid in late 1971. The ceremony included a wrist cutting exchange of blood to seal their bond. The marriage was not considered legal, but Marisa Mell said, "Who cares? In our minds it will be real." According to Marisa Mell, their affair was so intense that while living in Rome they made a trip to the Italian town of Sarsina for a ritual exorcism at the Cathedral of St. Vicinius. Boyd abruptly broke off the affair after the intensity apparently became too much to bear. Marisa Mell in early 1972, after Boyd’s departure, had this to say about the break-up of their relationship. It was not your usual explanation. “We both believe in reincarnation, and we realized we’ve already been lovers in three different lifetimes, and in each one I made him suffer terribly.” For her part, Mell would fondly remember Boyd many years later in her autobiography 'Cover Love' from 1990, dedicating a whole chapter to their affair.