Kenneth Joseph Rankin was born in Manhattan on February 10, 1940, and grew up in the Washington Heights neighborhood, where he heard Afro-Cuban music, jazz, Top 40 radio and Brazilian records. His mother sang at home and introduced him to music, and he liked to say that his career began in the fourth grade, when he sang "O Holy Night" in a school Christmas play. He signed with Decca as a teenager and made a handful of singles from the late 1950s, then moved to Columbia, which issued his singles between 1963 and 1966. In Greenwich Village in 1960 he met the songwriter Laura Nyro, whom he later named as the single biggest influence on his musical life.

In 1965 he is credited as one of the guitarists on Bob Dylan's Bringing It All Back Home, the album on which Dylan moved to an electric sound. Soon afterward he made the first of what became more than twenty appearances on The Tonight Show, where host Johnny Carson liked him enough to write the liner notes for his first album, Mind-Dusters (Mercury, 1967). That record contained "Peaceful," a Rankin song that became a British hit for Georgie Fame in 1969, reaching No. 16, and then a bigger one for Helen Reddy in 1973, when it went to No. 2 on the American adult contemporary chart and No. 12 on the pop chart in both the United States and Canada. A second Mercury album, Family (1969), took in Beatles songs and carried photographs by Richard Avedon.

The 1970s were his peak recording years. At Little David Records, the label that also recorded the comedian George Carlin, he made Like a Seed (1972), Silver Morning (1974), Inside (1975) and The Kenny Rankin Album (1977). Carlin took him on the road from 1972, and the two toured together on and off for nearly ten years. Silver Morning carried the Beatles covers "Blackbird" and "Penny Lane," which became his best-known interpretations; Paul McCartney asked him to perform his Beatles songs when McCartney and John Lennon were inducted into the Songwriters Hall of Fame in 1987. The Kenny Rankin Album was a record of standards and ballads with Don Costa conducting a large orchestra. Singers from the jazz world had meanwhile begun taking his songs: Peggy Lee recorded "In the Name of Love," and Carmen McRae and Mel Tormé both recorded "Haven't We Met," written with Ruth Batchelor.

After After the Roses (Atlantic, 1980), produced by Arif Mardin and Costa, he recorded only sporadically through the 1980s and toured heavily; Hiding in Myself (Cypress, 1988) was produced by Rankin with Jeffrey Weber. He sang backing vocals on Michael Franks's Passion Fruit (1983), and appears on albums by Joni Mitchell (Wild Things Run Fast, 1982) and Art Garfunkel (Lefty, 1988). The turn toward jazz settings came in the 1990s. Because of You (Chesky, 1991), produced by Rankin and David Chesky, put his voice and guitar with a small group led by pianist Danilo Pérez, with bassist David Finck, drummer Dave Ratajczak, George Young on reeds and Steve Kroon on percussion, and included "'Round Midnight," "Someone to Watch Over Me" and the Brazilian pieces "Berimbau" and "Doralice." Professional Dreamer (Private Music, 1995) was a set of standards, and Here in My Heart (Private Music, 1997), recorded principally in Rio de Janeiro, took up Brazilian songs by Ivan Lins and Antonio Carlos Jobim with Michael Brecker, Ernie Watts, Don Grusin, Toots Thielemans, Brian Bromberg, the guitarist Oscar Castro-Neves, and singers Rosa Passos and Bebel Gilberto. In between he was a guest vocalist on both volumes of Benny Carter's Songbook (MusicMasters, 1996 and 1997), and he sang on albums by Dan Siegel (Going Home, 1991) and David Benoit (Shaken, Not Stirred, 1994).

The last stretch of his recording career was the most openly jazz-oriented. He released a live set from the Bottom Line, The Bottom Line Encore Collection (1999, taped in 1990), and a self-produced A Christmas Album (1999) with guitarist Joe Beck and bassist David Finck. Reviewing a 2000 performance, the critic Don Heckman wrote in the Los Angeles Times that Rankin had been, for a decade or more, a singer whose improvisational skill and sense of swing made him a consistently appealing jazz artist. A Song for You (Verve, 2002), produced by Tommy LiPuma and Al Schmitt, paired him with Christian McBride on bass, Lewis Nash on drums, guitarist David Spinozza, orchestrations by Alan Broadbent and John Beasley, and Hubert Laws among the woodwinds. He brought a cabaret show of the same name to Feinstein's at the Loews Regency in New York in 2003 and was still performing in Los Angeles in 2008.

Rankin was married to Yvonne Rodriguez-Calderone. His 1970s records credit a Yvonne Rankin as co-writer, background vocalist and producer. In the spring of 2009 he was preparing an album of new material with producer Phil Ramone for Mack Avenue Records when he became ill. He passed away at Cedars-Sinai Medical Center in Los Angeles on June 7, 2009, at 69, of complications from lung cancer that had been diagnosed about three weeks earlier. He was survived by his son, Chris, his two daughters and a granddaughter.

Rankin sang in a light, high, supple tenor and accompanied himself on guitar, and the influences he named were mixed: the Brazilian and Afro-Cuban records of his New York childhood, the Beatles, whose songs he made a specialty, and Laura Nyro, who he said changed his musical life more than anyone. His career sits openly on the jazz/pop border. AllMusic files him under pop/rock, folk and vocal music, calls him a pop and standards singer, and notes that he returned to his singer-songwriter roots on After the Roses; most of his recording life was spent in pop. The jazz side is nonetheless documented: Carmen McRae, Mel Tormé and Peggy Lee all recorded his songs, he made records with Christian McBride, Michael Brecker and Danilo Pérez, and a JazzTimes review of Here in My Heart compared his intimate delivery and tender embellishment of melodies to Chet Baker's.