Paul Louis Hardcastle was born in Kensington, London, on December 10, 1957. AllMusic's biography says he came to keyboards as a teenager listening to Hawkwind, Deep Purple and Black Sabbath, that a motorbike racing accident left him in hospital with compound fractures, and that he then traded a video camera for a small synthesizer and began experimenting. In 1981 he joined the London soul band Direct Drive as keyboard player and appears on their singles "Don't Depend on Me" and "Time's Running Out." With Direct Drive's singer Derek Green he left in 1982 to form First Light, whose first single, a reworking of America's "A Horse with No Name," came out on Charlie Gillett's Oval label in June 1982. Their club hit "Sixteen Minutes of First Light" followed in November, and "Explain the Reasons" and "Wish You Were Here" reached numbers 65 and 71 on the UK chart in 1983 and 1984.

In March 1984 he released his first record under his own name on his own Total Control label, a medley of D-Train's "You're the One for Me" with his "Daybreak" and "A.M.," which reached number 41 in the UK. The instrumental "Rain Forest," an electro-funk track issued on Bluebird in 1984, also stalled at number 41 in the UK, but Profile Records picked it up in the United States, where it reached number 57 on the Hot 100, number 2 on the dance chart and number 5 on the soul chart. The Rain Forest album (Profile, 1985) was nominated for Best R&B Instrumental Performance at the 28th Annual Grammy Awards, where it lost to Ernie Watts's Musician.

"19" arrived in late April 1985. He built it from narration and interview fragments taken from the ABC documentary Vietnam Requiem, with a stuttering sampled hook that came from the two-second limit of the E-mu Emulator he was using. The title refers to the documentary's claim about the average age of an American combat soldier in Vietnam, a figure later disputed. It entered the UK chart at number four, spent five weeks at number one, reached number 15 on the Billboard Hot 100 and number 1 on the US dance chart, and was the best-selling single of 1985 in 13 countries. Versions with newsreaders speaking French, Spanish, German and Japanese followed, and it won the Ivor Novello Award for best-selling single of 1985. His self-titled Chrysalis album (November 1985) reached number 53 in the UK and was certified Silver by the BPI. Its other singles were "Just for Money" (number 19, co-written with Kim Fuller, with spoken contributions credited to the actors Bob Hoskins and Laurence Olivier) and "Don't Waste My Time" (number 8 in March 1986), sung by Carol Kenyon. His next hit, "The Wizard," was written for the BBC and served as the theme of Top of the Pops from 1986 to 1991. He also remixed for other artists in these years, with credits that include Hiroshima's "One Wish," D-Train, Change, Barry White and Ian Dury and the Blockheads, and he co-produced the 1986 charity single "Give, Give, Give" by Disco Aid.

His last UK chart entry under his own name was "40 Years," in the summer of 1988, from the album No Winners. After that his work was less visible in Britain and more successful elsewhere. An All About Jazz biography credits him with number one singles in Japan such as "Don't Be Shy" and "Blue Days," and much of his 1990s catalog was issued in Japan by JVC (Victor). Under the name The Deff Boyz he released the 1990 single "Swing," which reached number 84 in the UK, number 27 in Germany and number 18 on the US dance chart. With the singer Jaki Graham he made two albums as Kiss the Sky for Motown and JVC (1991 and 1994).

The jazz side of his catalog begins in the early 1990s with The Jazzmasters, a series he started on his own Fast Forward label and then ran through JVC and his own Hardcastle Records and Trippin 'N' Rhythm Records. AllMusic describes the first album as Hardcastle playing most of the instruments, with the saxophonist Gary Barnacle and the vocalist Helen Rogers, and says it stayed on Billboard's Contemporary Jazz albums chart for more than a year. A parallel numbered series under his own name began with Hardcastle (1994, JVC), which Wikipedia's discography, citing Billboard, lists at number 2 on the jazz albums chart; Hardcastle 2 (1996) reached number 7, Hardcastle 4 (2005) number 2 and The Jazzmasters VII (2014) number 1. The Chill Lounge volumes (2012 to 2015) and Moovin & Groovin (2014) also charted. The same biography says he has topped Billboard's smooth jazz chart 22 times, a self-reported figure; AllMusic notes that the 2022 single "Wavelength" was a number one on Billboard's jazz singles chart. In 2011 he released "Rainforest / What's Going On," pairing his best-known instrumental with Marvin Gaye's vocal after, according to the same biography, almost a year of negotiations with the Gaye family.

The later records lean on family and regular collaborators: his son Paul Hardcastle Jr. on saxophone, his daughter Maxine Hardcastle as a vocalist and DJ, the saxophonist Rock Hendricks, who is featured on "Happy Go Lucky" (2019) and "Wavelength," and Ryan Farish, with whom he made the 2011 Transcontinental EP. Hardcastle 8 (2018) includes a remake of the First Light version of "A Horse with No Name." Hardcastle X (The Eclipse) appeared in February 2022, reached number 14 on the jazz albums chart, and is the latest entry in the numbered series that AllMusic and Wikipedia list. A four-disc Chrysalis box set of his 1984 to 1988 work, Nineteen and Beyond, appeared in 2023. He remains active: Apple Music lists a new single, "The Groove In You," on Trippin 'N' Rhythm Records dated August 27, 2026, and earlier singles including "Rainforest 40" (October 2024) and a 2025 remix of "19" made with Paul Hardcastle Jr. Paul Hardcastle Jr., a saxophonist and DJ, died in January 2026 at the age of 35. Hardcastle married Dolores Baker in 1985 and they have three children.

Hardcastle is a keyboardist, programmer and producer who plays most of the parts on his own records, and the plain description is that he is a synthesizer-based dance and electro-funk producer who became a smooth jazz fixture. AllMusic's biographer notes that, although he is often associated with smooth jazz, his synth-based style is closer to instrumental dance music and electro-R&B, with occasional vocal and saxophone flourishes, and it files him under Contemporary Jazz, Crossover Jazz, Fusion and Electro. He has said of "19" that its roots are in electro, Afrika Bambaataa in particular, to which he "added a bit of jazz and a nice melody." The jazz connection is therefore one of idiom, audience and radio format rather than improvisation: his records are programmed, arranged and sequenced, with saxophone, guitar and voices added over his keyboard parts. A 2013 review on All About Jazz observed that there is essentially no difference between his solo and Jazzmasters albums and that the same musicians appear on both, which is the clearest description of how the two series relate. An artist biography published by All About Jazz describes the Jazzmasters sound as the smooth feel of "Rain Forest" blended with a Kenny G approach. As a television-theme and chart-instrumental composer who works with electronic sounds he can be set beside Jan Hammer, whose Miami Vice theme went to number one in the United States in 1985, the year of "19."