"Pedro broke the mould," he added. "I had to break it. All the trash I've done. I just couldn't do it anymore". While filming in Mexico, Gavin heard Universal was making an expensive 1920s-era Julie Andrews musical Thoroughly Modern Millie (1967) for George Roy Hill, again for Producer Ross Hunter. He lobbied for the role of Mary Tyler Moore's stuffy boyfriend to Hunter and Universal production head Ed Muhl. "This is a square, square guy so I told them it would be such type casting that they just couldn't get anyone else but me", said Gavin.

During the Korean War, Gavin was commissioned in the U.S. Navy serving aboard the USS Princeton offshore Korea where he served as an air intelligence officer from 1951 until the end of the war in 1953. Due to Gavin's fluency in both Spanish and Portuguese, he was assigned as Flag Lieutenant to Admiral Milton E. Miles until he completed his four-year tour of duty in 1955. He received an award due to his work in the Honduras floods of 1954.

Universal groomed Gavin as a virile, strapping leading man in the mould of Rock Hudson. They gave him roles in the films Behind the High Wall (1956), Four Girls in Town (1957), and Quantez (also 1957). He was meant to star in The Female Animal (1958) but was too busy on other projects and was replaced by George Nader. Gavin later recalled, "When I started out in front of the cameras I was green – raw, scared and just plain awful".

Gavin married Actress Cicely Evans in 1957. They had two children and lived in Dennis O'Keefe's former house in Beverly Hills. Gavin's first marriage ended in divorce in 1965. While making No Roses for Robert in Italy in 1967 he dated co star Luciana Paluzzi.

Gavin's first big break was being given the lead in A Time to Love and a Time to Die (1958), directed by Douglas Sirk from the novel by Erich Maria Remarque. This drew comparisons with the casting of the similarly-inexperienced Lew Ayres in Universal's film version of All Quiet on the Western Front (1931).

In 1960, Hedda Hopper claimed she suggested Gavin play the lead in Back Street over william Holden or Gregory Peck as he was "a better actor than either of them."

Gavin was cultural adviser to the Organization of American States from 1961 to 1965.

Gavin left Universal in 1962 to freelance. He signed to make several movies in Europe including The Assassins, The Challenge and Night Call. However he pulled out of The Assassins (which became Assassins of Rome (1965), Night Call was never made and The Challenge kept getting pushed back and was eventually permanently shelved.

In September 1964, Gavin signed a new contract with Universal which gave him the option to take work outside the studio. He tried another TV series, Convoy which only had a short run before being cancelled. Gavin then appeared in a Mexican film Pedro Páramo (1967), based on the novel by Juan Rulfo. "I had to do something I was proud of", said Gavin of the latter movie.

Gavin had been on the Board of the Screen Actors Guild (SAG) in 1965. He served one term as Third Vice-President, and two terms as First Vice-President. He was President from 1971-73. According to the SAG website:

In June 1966, Gavin signed a new non-exclusive contract with Universal, for five years at one film per year. Gavin did not regain his former prominence but was cast in the lead in OSS 117 – Double Agent (1968), then titled No Roses for Robert, replacing Frederick Stafford (who was filming Alfred Hitchcock's Topaz). He won good supporting roles in The Madwoman of Chaillot (1969) and Pussycat, Pussycat, I Love You (1970), in which he parodied his own image).

Gavin was also President of Gamma Holdings, a global capital and consulting company which he helped found in 1968. He was chairman of Gamma Services International from January 1990.

In the late 1970s Gavin concentrated on TV and his growing Business interests. His best-known performance around this time was playing Cary Grant in the TV movie Sophia Loren: Her Own Story (1980).

Gavin was signed for the role of James Bond in the film Diamonds Are Forever (1971) after George Lazenby (Bond in the previous series entry) left the role. "Time was getting awfully short", said Producer Albert Broccoli. "We had to have someone in the bullpen". Head of United Artists, David Picker, however, wanted the box-office insurance of Sean Connery, and made Connery a highly lucrative offer to return as Bond. Gavin's contract was still honored in full. According to Roger Moore's James Bond Diary, Gavin also was slated to play Bond in Live and Let Die (1973), but Harry Saltzman insisted on a British actor for the role and Roger Moore played the role instead.

In 1973, Gavin replaced Ken Howard in the Broadway musical, Seesaw opposite Michele Lee, beating out Tab Hunter who also auditioned because, according to the producers, Gavin "sings and dances better than Hunter and looks great on stage with Michele". (Gavin later claimed he was offered the musical from the beginning but turned it down because the book was not up to scratch, then changed his mind when Michael Bennett asked him to join the cast later.) He played the role for seven months, then stayed in it when the show toured the United States with Lucie Arnaz. Both the Broadway and touring production were directed by Michael Bennett. The Los Angeles Times said he gives a "solid performance".

Gavin was married to Constance Towers, a stage and television Actress, from 1974 until his death. The couple first met in 1957 at a party when his godfather, Jimmy McHugh, introduced them. Towers had two children from her previous marriage to Eugene McGrath. Gavin's elder daughter, Cristina, followed in his footsteps and became an Actress. His younger daughter, Maria, also followed in Gavin's footsteps, with a master's degree from Stanford, and has a successful career in television production.

Gavin had numerous Business interests parallel to his acting career. In June of 1986 following his work as ambassador to Mexico, Gavin became vice-president of Atlantic Richfield in the field of federal and international relations. In 1987 he resigned to become President of Univisa Satellite Communications, a new subsidiary of Univisa, the Spanish language broadcasting empire. He worked with them until December 1989.

In 1991, Gavin was sounded out about running for the Senate for the Republican Party but decided not to.

He was Senior Counselor to Hicks Trans American Partners (a division of Hicks Holdings) from 2001, a Managing Director and partner of Hicks, Muse, Tate & Furst (Latin America) from 1994 to 2001. He was an Independent Trustee of Causeway International Value Fund since September 2001.

Before A Time to Love and a Time to Die had been released, Gavin had already been cast by Douglas Sirk in another important role – supporting Lana Turner in Imitation of Life (1959). Unlike A Time to Love and a Time to Die, this was a spectacular success at the box office, and Gavin was voted most promising male newcomer for his performance in the film by the Motion Picture Exhibitor.

Gavin died at his home in Beverly Hills, California on February 9, 2018 of complications from pneumonia. He was also reported to have been fighting leukemia for an undisclosed period of time. Gavin was 86.