Hewson studied at the Guildhall School of Music, of which George Martin was also an alumnus from the previous generation. Although his education was in classical music, Hewson also had a strong love for jazz and big-band swing, and played guitar in a trio that included Peter Asher, one half of the duo of Peter & Gordon and also the brother of actress Jane Asher, who was romantically associated with Beatles bassist/singer Paul McCartney for most of the 1960s; it was on the occasions when the trio practiced at the Asher family home that Hewson first met McCartney socially. Later on, when the Beatles' bassist was recording Mary Hopkin's single "Those Were the Days," he decided that the song required orchestral accompaniment and on McCartney's instructions to get an arranger, Asher (by then an executive at the Beatles' Apple Records label) brought Hewson in to finish the record, in what he admits was his very first job after graduating from music school.

The record was a huge success, with a simple but distinctive arrangement that included the cembalon, a dulcimer-like instrument of Hungarian origin. Hewson later orchestrated Hopkin's debut album, Post Card, and her single "Goodbye," for which he utilized a string orchestra of a dozen violas and also arranged the orchestra for James Taylor's debut album on Apple. In 1970, he was engaged to orchestrate several of the songs on the Beatles' Let It Be album, including "The Long and Winding Road" and "I Me Mine." As related by Hewson to Matt Hurwitz in an interview for Good Day Sunshine magazine, it was under the prodding of producer Phil Spector that he engaged the vast, outsized orchestra that completed "The Long and Winding Road" in a manner that the Beatles hated but which the public loved.

By this time, Hewson's burgeoning experience as an arranger and conductor, coupled with a list of clients that included the Beatles, and with a handful of huge hit recordings to his credit, was getting him work outside of the orbit of Apple Records. His first engagement to score a movie came with the David Puttnam production Melody (also known as S.W.A.L.K.), starring Mark Lester, Tracy Hyde, and Jack Wild, in which he composed incidental background music and arranged orchestral versions of the three Bee Gees songs used in the film.

Hewson was contacted by McCartney during the spring of 1971 and engaged to prepare an instrumental version of the Ram album, which had not yet been released, for orchestra and chorus. That project, recorded in June of 1971 -- and held in the tape library under Hewson's name -- would eventually see the light of day as the Thillington album six years later, credited to Percy Thrillington. In 1972, he was brought in once more by McCartney, now leading the group Wings, to arrange the orchestral accompaniment on the single "My Love." By the middle of the 1970s, Hewson was working regularly as a conductor/arranger on projects involving various rock groups, including the Island Records band Bronco, Supertramp (beginning with Crime of the Century), Al Stewart (Past, Present & Future), Renaissance (Ashes Are Burning, Scheherazade & Other Stories), Carly Simon ("Nobody Does It Better"), Ray Thomas (From Mighty Oaks), and Diana Ross (Baby, It's Me).

He also began home recordings of his own under various aliases, including the RAH Band and Key West, through which he had hit dance singles with "The Crunch" and "Looks Like I'm in Love Again," respectively. In the decades since his first plunge into pop music arranging, Hewson has kept busy working with artists including Nick Drake, Clifford T. Ward, Hank Marvin, ex-Renaissance vocalist Annie Haslam, Colm Wilkinson, Cliff Richard, and Elliott Murphy; he was also the arranger and conductor on the soundtrack to the 1980 movie Xanadu, starring Olivia Newton-John and Gene Kelly. Along with George Martin and Paul Buckmaster (of "You Can't Always Get What You Want" fame), he remains one of the most visible of classically trained participants in the pop and rock music fields.