Seeking a new manager, Ray wrote to Gordon Mills, who had helped launch Tom Jones and Engelbert Humperdinck to international success. Mills was impressed by the demo tape enclosed and agreed to manage Ray. Just as he had done for Jones and Humperdinck, Mills saw the need for a clever name for his newfound talent. Playing off Ray's real last name and the playwriting team of Gilbert and Sullivan, the name Gilbert O'Sullivan seemed a natural choice. Ray hated it, but agreed that, when it came to marketing, Mills knew best.

The debut album, "Nothing Rhymed", on the new MAM label, featured some clever lyrics and strong melodies, and reached the UK Top 10 in late 1970. Just as Mills had planned, audiences were amused by O'Sullivan's image. His pudding basin haircut, short trousers and flat cap made him look like a schoolboy from the 1960s. Ray was slow to adopt the attitude of a star. He put himself completely in the hands of his manager and started living on the grounds of Mills' Weybridge house on a meager 10 pound-a-week allowance. His hit-making potential was undeniable and his voice and song style reminded many of Paul McCartney. The hits songs kept coming in Britain with, "We Will" (#16) and "No Matter How I Try" (#5).

Ray's big international break came in the Summer of 1972 when "Alone Again (Naturally)" gained the attention of audiences in North America. The song quickly rose up the charts, peaking at #1 and selling over a million copies. His American debut album, "Himself" was also highly accomplished and included the radio favorite "Matrimony", which would have provided a sizeable hit if his record company had the sense to release it as a single. O'Sullivan went on to become one of the biggest selling artists of 1972, following with two consecutive hits, "Get Down" (#7) and "Clair", both of which were number ones in Britain. While living with Gordon Mill's family, Ray became close to Mill's young daughter, Clair, who was the inspiration for the hit song. That's actually her giggling at the end of the record.

By this time, O'Sullivan's image had radically changed and he began to appreciate the superstar trappings enjoyed by Mills' other acts. His second album, "Back To Front", reached #1 in the UK and his appeal stretched across the board, embracing teen and adult audiences. For a time, he seemed likely to rival and even excel Elton John as Britain's most successful singer/songwriter export.

Although further hits were forthcoming with "Ooh Baby: (#25 US, #18 UK), "Happiness Is You And Me" (#19 UK) and "Christmas Song" (#12 UK), it was evident that his appeal had faded by the mid-70s. Following the UK Top 20 hit "I Don't Love You But I Think I Like You" in the Summer of 1975, Ray couldn't seem to crack the record charts anymore. After a public falling out with Mills, he left MAM and returned to CBS, the label that had launched his career. After five years of releasing failed recordings, only one hit, "What's In A Kiss? (#19 UK), emerged. Without the genius of Gordon Mills behind him, it seemed that the superstar of the mid-'70s was incapable of rekindling his once illustrious career. His disillusionment culminated in a High Court battle against his former manager and record company which came before justice Mars Jones in the Spring of 1982. The judge not only awarded O'Sullivan substantial damages and had all agreements with MAM set aside, but decreed that all the singer's master tapes and copyrights should be returned. The case made legal history and had enormous repercussions for the British music publishing world. Despite his court victory over Mills, O'Sullivan failed to re-establish his career to its former glory. He spent the 1980s moving to New Jersey and recharging his batteries.

O'Sullivan was back in the news as a result of a 1991 court case in which he successfully sued rapper Biz Markie over the rights to sample "Alone Again (Naturally)". The 1990s proved to be a better decade for Ray, scoring a #1 single in Japan and releasing albums that have done well internationally, including "Live in Japan" (1993), "By Larry" (1994), "Every Song Has It's Play" (1995) and "Singer Sowing Machine" (1997).

As the new millennium came, O'Sullivan continued to tour and record. His next chart appearance came in 2004 when another compilation album, "The Berry Vest of Gilbert O'Sullivan", made the UK Top 20. His album "A Scruff At Heart" was released on April 23rd 2007, featuring "Just So You Know", his first downloadable track. July '08 saw his next single release, "Never Say Di". That same year he appeared at the Glastonbury Festival and performed at London's Royal Albert Hall on October 26th, 2009. O'Sullivan landed on Hypertension records on August 26th, 2010, joining Leo Sayer, Chris DeBurgh, Fleetwood Mac and Gerry Rafferty. His first effort for his new label was called "Gilbertville", issued on January 31st, 2011. That album included the track "All They Wanted To Say", which dealt with the events of 9/11. On July 19th he played live on the BBC Radio 2 Ken Bruce Show and a month later, the documentary Out On His Own was broadcasted on BBC4. His 2012 touring schedule had him making numerous appearances across the UK. On April 23rd, 2015, O'Sullivan announced his latest album of new material, "Latin ala G" would be released on June 8th, followed by an eight date tour of the UK. The Spring of 2017 saw a busy touring schedule of shows across England. 2018 brought a well received, self-titled album produced by Elton John.

It wasn't until July 9th, 2019 that O'Sullivan was finally slated to return to America for the first time in forty-three years. The performer explained his absence to the press by saying that at the height of his career, his management chose to headline across Europe instead of supporting The Moody Blues on a U.S. Tour. More European dates were set to fill the remainder of the year, but plans to return State-Side for 2020 were already in the works.