William Morrison Charlap was born in New York City on October 15, 1966, into a musical family. His father, Moose Charlap, was the Broadway composer best known for the 1954 musical Peter Pan, and his mother, Sandy Stewart, was a singer and a regular on Perry Como's Kraft Music Hall whose recording "My Coloring Book" reached number 20 on the Billboard chart in 1963. Moose Charlap died in 1974, when his son was seven. Charlap began piano at three, studied classical music alongside jazz, attended New York's High School of Performing Arts, and left college without finishing in order to practice and work. He is a distant cousin of the pianist Dick Hyman.

His first steady jobs came in the late 1980s. The pianist Bill Mays recommended him as a replacement in the Gerry Mulligan Quartet, and he first recorded with Mulligan's group in 1989. Through the early 1990s he also worked with Benny Carter, Clark Terry and Frank Wess and became a sought-after accompanist for singers, among them Tony Bennett, Carol Sloane and Sheila Jordan. In the mid-1990s he was musical director of Midnight in the Garden of Good and Evil, a Johnny Mercer tribute presented as part of New York's JVC Jazz Festival. In 1995 he joined the Phil Woods Quintet, where he was the pianist on a run of recordings that continued into 2008. His own first album, Along with Me, was recorded between 1991 and 1993 and issued by Chiaroscuro in 1994, followed by Souvenir (Criss Cross, 1995) and Distant Star (Criss Cross, 1997).

In 1997 he first recorded with the trio that has been his main vehicle since, with bassist Peter Washington and drummer Kenny Washington (the two are unrelated despite the shared surname). Their first album together, All Through the Night, was recorded in 1997 and released on Criss Cross in 1998. He signed with Blue Note in 2000 and made Written in the Stars for the label that year, then built a run of albums each centered on one composer: Stardust (2002, Hoagy Carmichael, with guests including Tony Bennett, Frank Wess, Jim Hall and Shirley Horn), Somewhere: The Songs of Leonard Bernstein (recorded 2003, released 2004), and Plays George Gershwin: The American Soul (2005). The live Live at the Village Vanguard was recorded in 2003 and released in 2007. The trio's Somewhere, Live at the Village Vanguard and Uptown, Downtown (Impulse!, 2017) were each nominated for the Grammy Award for Best Jazz Instrumental Album, and the trio moved to Impulse! for Notes from New York (2016) before returning to Blue Note for Street of Dreams (2021) and And Then Again (2024), its second live recording from the Village Vanguard.

Singers have been a constant thread. He recorded Love Is Here to Stay (Blue Note, 2005) and Something to Remember (Ghostlight, 2012) with his mother, Sandy Stewart. His collaboration with Tony Bennett, The Silver Lining: The Songs of Jerome Kern, released on September 25, 2015, with Peter Washington and Kenny Washington as the rhythm section and Renee Rosnes as a guest on four tracks, won the Grammy Award for Best Traditional Pop Vocal Album at the 58th ceremony in February 2016. The trio then backed Bennett and Diana Krall on Love Is Here to Stay (Verve and Columbia, September 14, 2018), on which Charlap made the arrangements. In 2025 he and Dee Dee Bridgewater released the duo album Elemental on Mack Avenue (June 13, 2025). It was nominated at the 2026 Grammy Awards for Best Jazz Vocal Album, his fifth Grammy nomination. He has also recorded with Scott Hamilton, Harry Allen, Houston Person and Freddy Cole, and has performed with Wynton Marsalis.

Since 2001 he has recorded for the Japanese label Venus Records as a member of the New York Trio, with bassist Jay Leonhart and drummer Bill Stewart, in a long series of standards albums. In 2008 he joined the Blue Note 7, a septet assembled for the label's 70th anniversary with Nicholas Payton, Ravi Coltrane, Steve Wilson, Peter Bernstein, Peter Washington and Lewis Nash; their album Mosaic: A Celebration of Blue Note Records was released in 2009, with a tour of the United States from January to April of that year, and Charlap co-produced it and arranged Horace Silver's "The Outlaw." On August 25, 2007, he married the Canadian pianist Renee Rosnes, and the two have often played as a piano duo, including on Double Portrait (Blue Note, 2010).

Charlap succeeded Dick Hyman as artistic director of the Jazz in July series at 92NY in New York, a post he held from 2004 through 2023. He has produced concerts for Jazz at Lincoln Center, the New Jersey Performing Arts Center, the Chicago Symphony Center and the Hollywood Bowl, and Jazz at Lincoln Center gave him its Award for Artistic Excellence in 2024 at a gala honoring Tony Bennett. Since September 2015 he has been Director of Jazz Studies at William Paterson University in Wayne, New Jersey, and the university's faculty directory and its November 2025 announcement of his Grammy nomination still list him in that post.

Charlap is known above all as an interpreter of American popular song, and the composers he has built whole records around, Carmichael, Bernstein, Gershwin and Kern, mark his taste for the writers of the Broadway and Tin Pan Alley repertoire; he has called Gershwin "the American soul." His approach is lyrical and harmonically informed but swinging, and his trio, with the rhythm section of Peter Washington and Kenny Washington, has stayed together for close to three decades, and his work with singers, from Sandy Stewart and Tony Bennett to Dee Dee Bridgewater, reflects the same attention to lyrics and melody. His early years in the Mulligan and Phil Woods groups placed him in the post-bop mainstream, and reference works file him with the straight-ahead pianists of his generation. A classical grounding from childhood and a household built around theater and cabaret music, through both parents, lie behind his sense of song form.