His mother was a church pianist, and he initially studied classical and gospel music, but switched to jazz after hearing a Charlie Parker record.
In the 1950s and 1960s, he worked with Howard McGhee, Cecil Payne, John Coltrane, Dinah Washington and Billie Holiday, among others; in 1961, he also participated in the Living Theater's production of Jack Gelber's The Connection.
In the 1990s, Hewitt became a central figure at New York's Smalls Jazz Club; aside from playing there several nights a week, he sometimes also ended up using the walk-in refrigerator as a place to bunk when times were rough.
During his lifetime only one track of Hewitt's playing was released, a version of the Kenny Dorham tune "Prince Albert" on the compilation Jazz Underground: Live at Smalls (Impulse, 1998).
After Hewitt's death, however, recordings made by Luke Kaven began to surface on Kaven's Smalls Records label: the trio discs We Loved You, Not Afraid to Live, Fresh from the Cooler, and Out of the Clear Black Sky, and the quintet date Four Hundred Saturdays.