Most people who have seen Karl Denson play have seen him standing to Keith Richards' left. He has been the Rolling Stones' touring saxophonist since October 2014, which is the largest stage any jazz musician of his generation works on and the first line of nearly every article written about him. It is also, in the shape of his career, a late and secondary fact. Denson led four straight-ahead jazz albums for a German jazz label before he ever led anything else, made a leader date for Blue Note, co-founded the band that put 1960s soul-jazz back on American dancefloors, and in 2023 took a quintet of much younger jazz musicians into the Blue Note Jazz Club in New York for four nights. The stadium gig is the part of the résumé a general listener knows. It is not the part that explains him.

He was born in Santa Ana, California on December 27, 1956, the fifth of six children, and grew up in Orange County with older siblings who filled the house with Motown and soul. He started on cello in the fourth grade and moved to saxophone in the seventh, when an older brother gave up on the instrument and handed it down. His first band, Just Funk, came together in the eighth grade playing Jimi Hendrix, Santana and Buddy Miles. In 1973, as a tenth grader, he joined De Nada, a Mexican wedding band, and started earning sixty to seventy dollars a night. He went to Fullerton Junior College and then Cal State Long Beach, intending at first to study veterinary medicine, and eventually left school for the jam sessions. His earliest documented appearance on a record is from that period: the Fullerton College Vocal Jazz Ensemble's Cloudburst (1977) lists a soloist named Karl Denson on one track. At twenty-three he joined the backing band of the R&B singer O'Bryan, touring for two years, appearing on Soul Train and playing the alto saxophone solos on two tracks of O'Bryan's You And I (1983). He formed the Karl Denson Quintet in Los Angeles in the early 1980s while delivering flowers for a living, and turned up on screen in 1988 as the saxophonist in the fictional band Sexual Chocolate in Coming to America, a part he reprised in the 2021 sequel.

The break came through Lenny Kravitz, whom Denson met at a Los Angeles session in 1988. The following year Kravitz called and asked him to play the solo on the single "Let Love Rule." He ended up playing saxophone on seven of that album's thirteen tracks, joined the touring band, and stayed through Mama Said (1991), where his horn is on four of fourteen, and the Are You Gonna Go My Way tour in 1993. When Kravitz dropped horns from the band, Denson left rock and started making jazz records. That is the pivot the Blue Note Records biography marks as the start of his recording career, and it happened in 1992, at thirty-five.

The four albums he cut for Minor Music, a German label, between 1992 and 1995 are the least-known part of his catalog and the strongest argument about what he is. Blackened Red Snapper (September 1992) is a post-bop record with Ron Stout on trumpet, Deron Johnson and Reg Webb splitting the piano chair, and John Patitucci playing bass on two of its nine pieces. Herbal Turkey Breast followed in 1993 with Nedra Wheeler on bass. Chunky Pecan Pie (1994) put him in front of a rhythm section of Dave Holland and Jack DeJohnette, with Pee Wee Ellis guesting on tenor on one track, and it is the record that ought to settle any argument about whether he can play this music. Baby Food (1995) is filed by Discogs under hard bop. In the same years he was the second horn on Fred Wesley's Minor Music trilogy, Comme Ci Comme Ça (1991), Swing & Be Funky (1993) and Amalgamation (1994), an apprenticeship with James Brown's trombonist and arranger that shaped everything he did next.

He moved to San Diego in 1993, and in 1994 co-founded the Greyboy Allstars with the DJ and producer Greyboy, the guitarist Elgin Park, the organist Robert Walter, the bassist Chris Stillwell and the drummer Zak Najor. West Coast Boogaloo (Greyboy Records, 1994), with Fred Wesley on trombone on three tracks, is the record that started the West Coast end of the acid-jazz revival, and the band's residency at San Diego's Green Circle grew the room from sixty people to two hundred and fifty. A Town Called Earth followed in 1997. Alongside it Denson made The D Stands For Diesel (Greyboy Records, 1995), a double LP with a five-piece horn section and the singer Andy Bey on two tracks, and played alto and tenor on Robert Walter's Spirit Of '70 (1996), an organ date with Gary Bartz. The Allstars came apart after roughly three years, and in 1998 Denson formed Karl Denson's Tiny Universe. The group was a hit at the 2000 Newport Jazz Festival and returned to play a full set there on August 11, 2002. In between came Dance Lesson #2, his Blue Note debut, released May 8, 2001 and recorded with a soul-jazz band of the old school: Chris Wood on bass, the guitarist Melvin Sparks, two organists in Leon Spencer Jr. and Ron Levy, E.J. Rodriguez on percussion and DJ Logic on turntables. The Bridge (2002) widened the frame again, with Fred Wesley and Hugh Ragin in the horns and Skerik on baritone.

The 2000s made him one of the most-called horn players in American music, largely because he could sit in almost any band without sounding like a guest. He is on eight of the twelve tracks of Stanton Moore's Flyin' the Koop (Blue Thumb, 2002) and co-wrote four of them. He plays flute on "Ideofunk" and saxophone on "Polo Towers" for The John Scofield Band's Überjam (Verve, 2002), two of that album's eleven pieces, in a band with Avi Bortnick on rhythm guitar and sampler, Jesse Murphy on bass, Adam Deitch on drums and John Medeski on keyboards. He appears on Roy Hargrove's RH Factor records, playing flute on one track of Hard Groove (2003) and tenor on one of the Strength EP (2004); on Randy Crawford's Naked And True (1995), saxophone on three tracks and flute on a fourth; on Steve Winwood's About Time (2003), saxophone on two pieces and flute on two more; and on Blackalicious's Blazing Arrow (2002), Victor Wooten's Palmystery (2008) and Robert Walter's 20th Congress's Get Thy Bearings (2013). His own records kept coming: Lunar Orbit (2007), made by a stripped-down Karl Denson Trio built on the Hammond B-3; Brother's Keeper (Shanachie, 2009), a septet with Meshell Ndegeocello on bass and a horn section including Michael Leonhart and Cochemea Gastelum; New Ammo (2014); and, with the Greyboy Allstars back together from 2003, What Happened To Television? (2007), Inland Emperor (2012) and Como De Allstars (2020).

In October 2014, Lenny Kravitz was at dinner with Mick Jagger when the subject of the Stones' critically ill saxophonist came up. Bobby Keys could not make the Australian and New Zealand leg of the 14 On Fire tour; Kravitz put Denson's name forward, and Denson's first show with the band was in Adelaide on October 25, 2014. Keys passed away on December 2 that year, and Denson has held the chair since, through the Zip Code tour of 2015 and six world tours, including the Havana concert of March 2016 that produced Havana Moon. He is on Sticky Fingers Live at the Fonda Theatre 2015 and Olé Olé Olé! A Trip Across Latin America. None of it displaced the rest of the work. Gnomes & Badgers, the Tiny Universe's most song-driven record, with Chuck Leavell, Lukas Nelson, Anders Osborne and Ivan Neville among the guests, came out on March 8, 2019. And in a year when the Tiny Universe turned twenty-five and the Greyboy Allstars turned thirty, Denson booked the Karl Denson Project into the Blue Note Jazz Club in New York for eight shows across four nights, February 23 to 26, 2023, with Keyon Harrold on trumpet, James Francies on piano, Burniss Earl Travis II on bass and Mike "Blaque Dynamite" Mitchell on drums. His explanation for the band was one sentence: he wanted to play with the young guys. They came back to the same room on October 3.

Denson is a big, blunt, physical tenor player with an unusually good flute, and the two halves of his listening are audible at once. The jazz side goes back through Fred Wesley, whose band he spent four years in, to the soul-jazz organ dates of the late 1960s, which is why Dance Lesson #2 hired Melvin Sparks and Leon Spencer Jr. rather than younger players who could imitate them. The other side is stated plainly in every account of the Tiny Universe: James Brown, Maceo Parker and Parliament-Funkadelic. What distinguishes him from the many horn players who list those names is that he came to them from the other direction, having already made four acoustic jazz albums, so the funk in his playing is a choice about rhythm rather than a limit on harmony. He describes his own current direction as jazz that is a little more dancey and a little more funky, and his working method as composition first: he writes songs as vehicles for improvisation, uses vocals to bring an audience in, and then plays over the form the way a jazz musician would. The Greyboy Allstars were the clearest expression of it. Denson's own claim for that band is that nobody else embraced West Coast boogaloo to the extent they did, and the record supports him, since the style barely existed as a going concern in 1994 and had a national club audience within three years. He is also a genuine doubler rather than a saxophonist who owns a flute: the flute is the lead voice on a large share of his own tunes, it is what Scofield and Roy Hargrove both called him for, and it is the instrument that separates his sound from every other tenor player on the jam-band circuit. AllMusic files him under jazz saxophone and post-bop; his own Discogs entry calls him a funk and jazz saxophonist, flutist and vocalist; and both descriptions are accurate, in that order or the other.