[9] He was unable to afford to attend music college because of a change in his parents' financial circumstances,[10] but had private lessons there for six months and developed his reading ability by playing with trombonist Morris Ellis' big band.

[6]: 34  He also developed by listening to Ahmad Jamal and others in clubs,[10] and "playing and practicing 12 hours a day" for the next five years,[6]: 34  but he remained self-taught as a pianist.

[11] Mabern learned orchestration techniques from bassist Bill Lee, and comping and chord voicing from pianists Chris Anderson and Billy Wallace.

According to his own account, he moved there with saxophonist Frank Strozier on November 21, 1959, checked in at a hotel and then went to Birdland, where he met Cannonball Adderley, who asked him if he wanted a gig.

Mabern accepted and was shown inside, where trumpeter Harry "Sweets" Edison, who was looking for a pianist to replace the soon-to-depart Tommy Flanagan, auditioned him and offered him the place.

[12][13] Mabern steadily built a reputation in New York as a sideman, playing with, among others, Lionel Hampton's big band in 1960 (including a tour of Europe),[14] the Jazztet for 18 months in the period 1961–62, accompanying vocalists, including Betty Carter, Johnny Hartman and Arthur Prysock, and working with trumpeter Donald Byrd and drummer Roy Haynes.

[15] From 1965, Mabern also worked with Freddie Hubbard, Jackie McLean, Hank Mobley, Blue Mitchell (1966), Sarah Vaughan, and Joe Williams (1966–67).

Mabern worked intermittently over a period of four decades with George Coleman, beginning in the 1960s, and including an appearance at the 1976 Newport Jazz Festival.

[5] Mabern's piano style was described as being "aggressive, very positive, crashing out chords that drop like pile drivers and warming up and down the keyboard with huge, whooping bursts of action", while, at the same time, he showed "a keen sensitivity" as "an extremely perceptive accompanist".

[29] Critic Gary Giddins identified some of the characteristics of Mabern's playing as being "blues glisses, [...] tremolos and dissonant block chords", that help to create a style "that marries McCoy Tyner's clustering modality with rippling asides that stem from [Art] Tatum".