At the age of 19, he co-founded Umbra, a group of young black writers and poets who played an important role in the birth of the Black Arts Movement – the artistic branch of the Black Power Movement. Based in the Lower East Side of Manhattan, Umbra organized weekly criticism and writing workshops and held public readings, soon to become very popular. The saxophonist Archie Shepp and the pianist Cecyl Taylor gave their support to this embryo of the Black Arts Movement. In 1963, this collective of young writers created a magazine, with Henderson as a co-editor, then editor. Amongst the voices that expressed themselves in the Umbra Magazine, there were those of Richard Wright, Alice Walker and Ishmael Reed.