Joseph Harry Fowler Connick Jr. was born in New Orleans on September 11, 1967. His father, Harry Connick Sr., was the district attorney of Orleans Parish from 1973 to 2003 and also played weekly gigs in French Quarter clubs. His mother, Anita Frances Livingston Connick, was a lawyer and judge who died of ovarian cancer when he was 13, and his parents owned a record store. He began learning keyboards at three, played in public at five and, at ten, recorded with a local Dixieland band: the debut album by Dixieland Plus, taped live in New Orleans on October 29 and 30, 1977, with trumpeter Roy Liberto, clarinetist Pee Wee Spitelera, trombonist Jim Duggan, bassist Placide Adams and drummer Freddie Kohlman, has Connick on piano and singing "St. James Infirmary" and "When the Saints Go Marching In." At nine he had played Beethoven's Third Piano Concerto with the New Orleans Symphony Orchestra, now the Louisiana Philharmonic, and as a boy he played a duet of "I'm Just Wild About Harry" with Eubie Blake at the Royal Orleans Esplanade Lounge. He studied at the New Orleans Center for Creative Arts and with the pianists Ellis Marsalis Jr. and James Booker.

After an unsuccessful stretch of jazz study at Loyola University New Orleans he moved to New York, where he attended Hunter College and the Manhattan School of Music and lived at the 92nd Street Y. There the Columbia Records executive George Butler persuaded him to sign with the label. Harry Connick Jr. (1987) was an instrumental record of standards and originals with Ron Carter on bass, including Thelonious Monk's "I Mean You" and Carter's "Little Waltz," and Connick dedicated "Sunny Side of the Street" to Booker. Regular dates at high-profile New York rooms built his name in jazz. 20 (1988) was his first Columbia album with vocals, six of its eleven tracks sung, and it carried a duet with Carmen McRae on "Please Don't Talk About Me When I'm Gone" and Dr. John on organ and vocals for "Do You Know What It Means to Miss New Orleans."

The director Rob Reiner heard a tape of Connick through the drummer Bobby Colomby and asked him to supply the music for When Harry Met Sally... (1989). The soundtrack album sets standards sung with a big band and orchestra arranged by Marc Shaiman against trio pieces with the bassist Ben Wolfe and the drummer Jeff "Tain" Watts, and it includes the tenor saxophonist Frank Wess and the guitarist Jay Berliner. It went double platinum, reached No. 1 on Billboard's jazz albums chart and won Connick the Grammy for Best Jazz Vocal Performance, Male. On July 3, 1990 he released two albums at once. We Are in Love was a big-band vocal album of mostly original songs, with Branford Marsalis on saxophones, Russell Malone on guitar, Wolfe on bass and Shannon Powell on drums; it also went double platinum and won a second consecutive Grammy in the same category. Lofty's Roach Soufflé was a trio instrumental album with Wolfe and Powell. Reviewers heard Thelonious Monk in the trio record's playing, and Connick answered that Duke Ellington had been the bigger influence. That year he also began touring with his own big band and made his film debut in Memphis Belle. Blue Light, Red Light (1991) gave the 14-piece band a set of his own songs, mostly with lyrics by Ramsey McLean, and 25 (1992) was a solo-piano collection of standards that brought in Ellis Marsalis on "Stardust," and Ray Brown and the tenor saxophonist Ned Goold on "On the Atchison, Topeka and the Santa Fe," and included John Coltrane's "Moment's Notice."

The commercial side grew alongside the jazz, and it is a large part of his public career. "Promise Me You'll Remember," his contribution to the soundtrack of The Godfather Part III, was nominated for an Academy Award and a Golden Globe. When My Heart Finds Christmas (1993) was the best-selling Christmas album of its year and the first of several holiday records. In 1994 he turned to New Orleans funk with a new band for She, followed by Star Turtle (1996); "(I Could Only) Whisper Your Name," from the soundtrack of The Mask, reached No. 67 in the United States and remains his best showing as a singles artist, and his 1994 British tour of the funk material left some fans who expected a jazz crooner unhappy. Acting ran in parallel: Memphis Belle (1990), Little Man Tate (1991), Copycat (1995), Independence Day (1996), his first leading role in Hope Floats (1998), and the voice of Dean McCoppin in The Iron Giant (1999).

To See You (1997) set his own love songs with a symphony orchestra, a format he toured in the United States and Europe and took to the Nobel Peace Prize Concert in Oslo. Come By Me (1999) returned to big-band jazz and was followed by a year-long world tour. In 2001 he wrote the score for Thou Shalt Not, Susan Stroman's Broadway musical after Émile Zola's Thérèse Raquin, which earned him a Tony Award nomination. That October he released 30, recorded in 1998, with Wynton Marsalis on trumpet on one track, and Songs I Heard, big-band reworkings of songs from children's films that won the Grammy for Best Traditional Pop Vocal Album. In 2002 he received U.S. patent 6,348,648 for a system for coordinating music display among the players of an orchestra, and he began a run as Leo Markus, Grace Adler's husband, on the NBC sitcom Will & Grace, which lasted through 2006.

From 2003 he made his instrumental records for Marsalis Music, the label run by Branford Marsalis, with Columbia's agreement, as a "Connick on Piano" series. Other Hours (2003) was a quartet date with Ned Goold on tenor saxophone, Neal Caine on bass and Arthur Latin on drums, playing twelve Connick pieces first written for Thou Shalt Not. Occasion (2005) paired him with Branford Marsalis in duo, and the two performed together at the 2005 Ottawa International Jazz Festival for the DVD A Duo Occasion. Chanson du Vieux Carré (2007) was a largely instrumental big-band album of New Orleans repertoire and brought two Grammy nominations, for composition and arrangement, for his piece "Ash Wednesday"; a fourth volume, Music from The Happy Elf, followed in 2011. On Columbia, Only You (2004), ballads from the 1950s and 1960s cut at Capitol Studios on a piano once used by Nat King Cole, reached No. 5 on the Billboard 200, his highest peak, and its PBS concert film won him an Emmy Award for Outstanding Music Direction. He starred in the 2006 Broadway revival of The Pajama Game, and Oh, My NOLA (2007) paired his big band with New Orleans songs, with Wynton Marsalis on trumpet and Jon Batiste on piano.

After Hurricane Katrina he became a prominent figure in the rebuilding of his hometown. He helped organize the NBC relief telethon of September 2, 2005, and with Branford Marsalis and Habitat for Humanity he developed the Musicians' Village in the Upper Ninth Ward, announced on December 6, 2005, whose centerpiece is the Ellis Marsalis Center for Music; the two shared the 2012 S. Roger Horchow Award for Greatest Public Service by a Private Citizen. He is also a founder of the Krewe of Orpheus, the music-oriented Mardi Gras krewe.

Your Songs (2009) was a collaboration with the executive Clive Davis, who chose most of the songs; Wynton and Branford Marsalis played on it, and it earned a Grammy nomination. Television took more of his time in the 2010s. He mentored finalists on American Idol in 2010 and 2013, sat on its judging panel for seasons 13 to 15, in 2014 to 2016, played a prosecutor in a four-episode arc of Law & Order: Special Victims Unit in 2012, and hosted the syndicated daytime talk show Harry from September 12, 2016 to May 23, 2018, earning Daytime Emmy nominations as host. Every Man Should Know (2013) was an album of all-original music. His second Emmy, in 2011, was for the concert special In Concert on Broadway.

In 2019 he moved to Verve for True Love: A Celebration of Cole Porter (October 25, 2019), for which he wrote the arrangements and conducted the orchestra, and took a Cole Porter stage production to Broadway; it returned for a five-day run at the Metropolitan Opera House in January 2025. Alone with My Faith (March 19, 2021) was made in his home studio during the COVID-19 lockdowns, with Connick arranging every song, playing every instrument and singing every part, and it earned a Grammy nomination for Best Roots Gospel Album. He played Daddy Warbucks in NBC's Annie Live! on December 2, 2021, released the Christmas album Make It Merry in 2022, was a judge on the 2023 revival of Australian Idol, and starred in the Netflix film Find Me Falling in 2024. On May 22 and 23, 2026 he made his Carnegie Hall debut with Babe: Elaboratio, a symphonic work dedicated to his mother, given on what would have been her 100th birthday, with Connick at the piano and Orchestra Victoria of Melbourne under Jessica Gethin. The three-part album, which he wrote, arranged and produced, was recorded in Melbourne with Orchestra Victoria and his big band and was released on September 25, 2026, and a companion book from Harper Celebrate followed on September 29. His career totals include three Grammy Awards from sixteen nominations and two Emmy Awards, and he continues to record and perform.

Connick's piano comes out of New Orleans teaching, above all Ellis Marsalis and James Booker, and he has honored Booker on record: "Sunny Side of the Street" on his debut is dedicated to him, and "Hear Me in the Harmony" on Star Turtle is a tribute to him. The early trio records invited comparison with Thelonious Monk, which he answered by pointing to Duke Ellington, and Chanson du Vieux Carré is full of New Orleans repertoire, from Louis Armstrong's "Someday You'll Be Sorry" to Professor Longhair's "Mardi Gras in New Orleans" and Sidney Bechet's "Petite Fleur." He is also an arranger who writes his own big-band charts and a bandleader whose touring band has drawn New Orleans players such as Lucien Barbarin, Leroy Jones and Jeremy Davenport alongside players from the New York jazz scene.

As a singer he is closest to the crooning tradition of Frank Sinatra. Reiner was struck by how much Connick sounded like a young Sinatra when he heard the tape for When Harry Met Sally..., and a Los Angeles Times review of We Are in Love heard a voice part Sinatra and part Mel Tormé. As of 2009 he had more number-one albums on Billboard's jazz albums chart than any other artist, a count that reflects how well his vocal and big-band records sold rather than a verdict on his piano.

His standing as a jazz musician has always been argued. AllMusic's Scott Yanow wrote of Lofty's Roach Soufflé that Connick had not yet found a piano style of his own, and Entertainment Weekly put Lofty's Roach Soufflé among the year's worst albums. His pop, funk, Christmas, Broadway and gospel records, and a screen career that made him widely known, sit beside the jazz and are not part of it. The records to hear for his playing without a voice or an orchestra in front are the 1987 debut, 25 and the Marsalis Music "Connick on Piano" volumes, and the big-band albums Blue Light, Red Light and Come By Me show the arranger and bandleader.