Kelly was born in Kingston in 1949. After leaving school, he spent a year studying electronics in Springfield, Massachussetts, United States during 1966, before returning to Jamaica. In 1967, when Slim Smith left The Techniques, Kelly was brought in to replace him, recording for Duke Reid in the rocksteady era when Reid's Treasure Isle studio/label was dominating Jamaican music. Kelly's falsetto voice, strongly influenced by the American Soul singer Sam Cooke, in combination with Winston Riley and Bruce Ruffin, maintained the success that The Techniques had enjoyed with Smith. The Techniques first record with Kelly, "You Don't Care", adapated from Curtis Mayfield's "You'll Want Me Back", spent six weeks at number one in the Jamaican singles chart, and was followed by further hits with "Queen Majesty", "My Girl", "Love Is Not a Gamble", "It's You I Love", and "Run Come Celebrate".
In 1968, Kelly went solo, moving from Reid to Bunny Lee, and debuting with another Mayfield cover, "Little Boy Blue".Kelly's "How Long Will It Take" was the biggest-selling Jamaican single of 1969, and was the first Jamaican record to feature a string arrangement, which was overdubbed when it was released in the UK on the Palmer Brothers' Unity label. An album, the Lee "Scratch" Perry-engineered Pat Kelley Sings (sic) followed, and Kelly was offered a £25,000 contract by Apple Records, which he was unable to accept due to existing contractual commitments. Kelly continued to record, having a big hits for producer Phil Pratt in 1972 with "Soulful Love" and "Talk About Love", and returning to record with Duke Reid, having another hit with a cover of John Denver's "Sunshine". He fell back on his earlier training, working as an engineer at Channel One Studios. He also moved into production, producing his own Youth and Youth album in 1978, and co-producing (with Holt) John Holt's The Impressable John Holt (Disco Mix) album in 1979. The late 1970s and early 1980s saw Kelly recording more regularly again, and he continued to record occasionally in the years that followed.
He is back on stage in 2009 with The Moon Invaders!