Walts has been described as an exemplary modernist, particularly as regards his covers for ''The Masses'', e.g. his illustration of Mary Fuller, April 1916. He was on the board of both ''The Massess'' and ''The Liberator'' and subsequently ''The New Masses''.
He is noted for his portrayal of African Americans, and his work was critically discussed by such people as W.E.B. Dubois who considered Walts "a thoughtful, clear-eyed artist" and defended him against critics.