Jenkins grew up in Penclawdd village in Gower (now part of Swansea), born to a Swedish mother and Welsh father, a chapel organist and choirmaster, who gave Karl his first music lessons. In high school, twelve-year-old Jenkins began playing oboe in the [a=National Youth Orchestra Of Wales]. He got formally educated at [l=Cardiff University], followed by postgraduate studies at the [l=Royal Academy of Music] in London.

Early in his career, Karl Jenkins was primarily known as a jazz and fusion/jazz-rock saxophonist, oboist, and keyboard player. He participated in several Graham Collier's ensembles and was active in an early free jazz/improv UK scene. In 1969, Jenkins co-founded [url=https://discogs.com/artist/184256]Nucleus[/url] band, which won the [l=Montreux Jazz Festival]'s first prize and headlined the [l=Newport Jazz Festival] the following year. As a session player and band member, Karl Jenkins appeared on such notable albums as Elton John's Tumbleweed Connection (1970) or Ode (1972) by Barry Guy with London Jazz Composers Orchestra. (Jenkins played alongside Mark Charig on a bugle and Alan Wakeman on tenor/soprano saxophones, cousin of Rick Wakeman of Yes fame).

In 1972, Karl Jenkins joined Soft Machine, a prog/jazz-rock/fusion band from Canterbury formed by Mike Ratledge. He remained with the group until 1978 (and joined for brief reunions in 1980–81 and '84), active as a lead composer on many Soft Machine LPs. Jenkins toured extensively with the band, performing at [l=Carnegie Hall], the [l=Royal Albert Hall]'s "Proms" and [l=The Reading Festival]. In November 1973, Jenkins and Ratledge participated in a live-in-the-studio performance of Mike Oldfield's Tubular Bells for the [l=BBC] (available on The Best Of Mike Oldfield Elements VHS/DVD compilation).

In the late 1980s and early '90s, Karl Jenkins mainly focused on commercial music. Collaborating with [url=https://discogs.com/label/1372991]Bartle Bogle Hegarty[/url] agency, Jenkins created soundtracks for [l=Levi's], [l=Renault], and De Beers, among other prominent clients. (Karl subsequently developed De Beers' theme into a full-length orchestral work, Diamond Music). In 1994, Jenkins started Adiemus — a series of orchestral new-age albums with vocals. (The opening song on the debut album, Songs Of Sanctuary, released in 1995 on CD/Cassette by [l=Virgin], was based on Karl's TV commercial tune for [l=Delta Air Lines].) Notably, all singing was in a fictional lyrical language, vaguely reminiscent of Latin, and constructed by Jenkins purely phonetically. (Similar to the Kobaïan language invented by French drummer and musician Christian Vander for [url=https://discogs.com/artist/179749]Magma[/url] band).

In 1999, Jenkins wrote The Armed Man: A Mass For Peace, a neo-classical Mass for orchestra and mixed choir, commissioned by the Royal Armouries and debuted at [l=Royal Albert Hall]. Since then, it become one of Karl's best-known works, with over 400 performances and several critically-acclaimed recordings.