Clemens Grassmann was born in Berlin in 1991 into a musical household. His father was a working guitarist, well known in West Berlin for radio, television and live work, and the guitars, keyboards and percussion instruments around the flat reached the son early. He started on drums at ten, after years of listening to his father's vinyl and cassettes in the car: Deep Purple with Steve Morse, Eric Johnson's "Cliffs of Dover," Huey Lewis, the Jackson 5 and Miles Davis's Kind of Blue. School music projects led to his high school big band, which is where the jazz took hold. He spent hours on film of Buddy Rich, Gene Krupa, Louie Bellson, Sam Woodyard and Papa Jo Jones, and reoriented his practice toward jazz and big band drumming.

The formal training came in two countries. In Berlin he entered a pre-study preparation programme and studied with Mario Wuerzebesser at the Berlin Jazz Institute, who rebuilt his technique and prepared him for conservatory auditions. He was admitted to the Hochschule fuer Musik Carl Maria von Weber in Dresden, where he read jazz, rock and pop drumming with Sebastian Merk and Michael Griener. In the summer of 2011 he spent two weeks in Umbria at the Berklee summer clinics and met Ron Savage, whose playing and teaching sent the next decade in a different direction. He took private lessons with Savage, won a scholarship, and moved to Boston in the fall of 2013 to enter Berklee College of Music.

He read for a Bachelor of Fine Arts in drum set performance and was an active member of the Berklee Global Jazz Institute from the fall of 2014 through the spring of 2016. The drum faculty at Berklee put him with Savage, Neal Smith, Kenwood Dennard, Billy Kilson, Ralph Peterson Jr. and Terri Lyne Carrington; through the Global Jazz Institute he took lessons with Danilo Perez, John Patitucci, Joe Lovano, Ben Street, George Garzone, Dave Liebman, David Gilmore and Kenny Werner. In March 2015 the school gave him its Armand Zildjian Percussion Award. He graduated in the spring of 2016.

His senior recital became his first record. Labyrinths & Tales, cut with what he called his Berklee gang at Soundscape Studios in Boston and released on January 19, 2017, is a seven-piece programme of his own writing for a shifting group: Jonathan Chapman on bass, Andrew Marzotto on guitar, Antoni Vaquer on piano, Justin Salisbury on synths, Negah Santos on percussion, Cosimo Boni on trumpet, Daniele Germani on alto, Alexandre Madeline on tenor, Xito Lovell on trombone and Logan Ivancik on baritone. Santos, who plays percussion on two of the seven pieces, went on to the house band of The Late Show with Stephen Colbert, and the record still appears in her published discography.

Brooklyn followed, and with it a second album built out of an accident. Booked at Rockwood Music Hall with a quartet of saxophone, guitar, bass and drums, Grassmann could not get a horn player and called a second guitarist instead. The situation repeated, he came to like it, and he wrote for the instrumentation deliberately: two guitars, two saxophones, bass and drums. MiDNiGHT APPLE was recorded in four hours at Big Orange Sheep in Brooklyn by Chris Benham, with Jernej Bervar and Yoav Eshed on guitars, David Milazzo on alto, Alex Madeline on tenor and Nick Dunston on bass, and released on April 13, 2019 with cover art, design and photography by Hollis King. Grassmann had met Dunston at the Father Knows Best jam session in Brooklyn and Milazzo at the Berklee session at Wally's in Boston. The title track takes its point of departure from Ahmad Jamal's "Poinciana" and then runs distorted guitar over it; "Sphere" sets the two saxophones in counterpoint and slips into 9/8; "Mystic" opens with a long solo bass passage and African-derived feels.

The third record came out of the pandemic. Grass Machine, released April 8, 2022, was cut at Rudy Van Gelder Studio in Englewood Cliffs, where Grassmann found the kit still set up the way Billy Hart had left it, in the room that produced John Coltrane's A Love Supreme, Herbie Hancock's Maiden Voyage and Wayne Shorter's Adam's Apple. The band is a septet: Aaron Bahr on trumpet, David Milazzo on alto, Sam Dillon and Chris Bittner on tenors, Chris McCarthy on piano and Cole Davis on bass. Grassmann wrote all six pieces and co-produced with Jannek Zechner. The title came from a nickname Milazzo had given him.

The sideman work runs parallel and is wider in idiom than the leader dates. His first session at Big Orange Sheep was not his own but Enrico Bergamini's debut Out of Place (2017), which All About Jazz gave four stars. He has an extended association with the Kuwaiti singer and guitarist Yousif Yaseen, playing on Visions (2016) and touring the Middle East with him for two weeks, and he toured the American South with the backbeat group Swanky Tiger, whose EP Mechanical Nightmares appeared in June 2017. He is the drummer on Seba Molnar's Level Up (2020), a record whose closing track brings in his former teacher George Garzone on tenor, and he appears on two pieces of Enrique Thompson's twenty-two-musician tango-jazz collection Ciclos (Artruism, 2021). He plays on Alex Madeline's Unexpected Direction (Lyonhill, 2024), including "Balance" with Sami Stevens singing and Chris McCarthy on keys, and he holds the drum chair on the Palestinian qanun player Firas Zreik's Tarabesque (Maqam Records, 2026). Alongside that he has worked steadily in New York's brass band scene with Brass Monkeys NYC, Underground Horns and Dingonek, in the singer Austin Giorgio's touring band across the United States and Europe, and with the Nicolas Delgado Group in Berlin. He teaches drums and percussion at every level and has held artist-in-residence posts in both Boston and New York.

Grassmann's stated aim is accompaniment rather than display: applying form and shape to a piece while staying receptive to what the other players are doing. The technical foundation is a specific and traceable one. He builds his practice on Alan Dawson's rudimental ritual, Ted Reed's Syncopation, George Lawrence Stone's Stick Control and Ralph Peterson's motion studies, keeping a foot pattern going and drilling the hands on top of it, and he credits Ron Savage with teaching him how to play rather than how to execute. Where he departs from a conventional timekeeping role is in comping: because the drums are not fixed to pitches, he treats them as a harmonic instrument, working out voicings, tensions, clusters and sustained pads to shadow a tune's changes. The Berklee term for it, harmonic rhythm, is the one he uses himself. Two of his teachers pushed him in opposite directions on the same question and he names both: Kenwood Dennard taught him to throw himself deliberately off balance to see what it would do to his playing, and Kenny Werner taught him to disconnect from the analysis and let the music come out of the body.

The writing is where the doubleness he keeps describing actually shows. Both later albums are built on pairs: two guitars against two saxophones on MiDNiGHT APPLE, two tenors flanking an alto and a trumpet on Grass Machine, call set against response, long tones against rhythmic figures, tradition against contemporary effects. His compositions pull from Ahmad Jamal on one track and a rocket leaving the atmosphere on the next, and he is unembarrassed about the electronic and Latin material sitting next to the swing. Reviewers have consistently placed him between camps rather than in one: Jazz Weekly called him "my kind of guy for the future of jazz" and credited him with modern and clever textures, JazzCorner described tracks that blur the lines between fusion and jazz, and the notice for Grass Machine in Making a Scene found the session straddling the line between mainstream and avant-garde and smoking throughout. His own listening runs to Joe Henderson, Nate Smith, Roy Haynes, Jackie McLean, Walter Smith III and Joe Lovano, and the drummers he names as forebears are Elvin Jones, Tony Williams, Alan Dawson, Frankie Dunlop, Philly Joe Jones, Max Roach, Art Blakey, Al Foster, Billy Higgins, Billy Hart and Jack DeJohnette.