A prolific black character actress of stage, screen, and especially television, Sinclair was long a fixture in drama series ("Trapper John, M. D." CBS, 1979-86, "Gabriel's Fire" ABC, 1990-91), miniseries ("Roots" ABC, 1977) and TV-movies ("I Know Why the Caged Bird Sings" CBS, 1979, "Guyana Tragedy: The Story of Jim Jones" CBS, 1980, "Divided We Stand" ABC, 1988). Sinclair's forthright manner has enhanced many tough but essentially warm-hearted character roles, some, like herself, of West Indies descent.
Among Sinclair's feature film credits are "I Will...I Will...for Now", "Cornbread, Earl and Me" (both 1975), "Leadbelly" (1976), "Star Trek IV: The Voyage Home" (1986) and "Coming to America" (1988). Her final film role was the voice of the Lion Queen in Disney's "The Lion King" (1994).
A capable and versatile businesswoman as well, Sinclair was an art dealer, chairwoman of the women's clothing manufacturer, Madge Walters Sinclair Inc., and owner of the Action Income Tax Service.