Born in New York City, Allen grew up in Harlem's Sugar Hill among neighbors that included Duke Ellington, Sonny Rollins, and Johnny Hodges, all of whom had an influence on her career choice. She made her Broadway debut at the age of six in The Wisteria Trees, Joshua Logan's Americanized adaptation of The Cherry Orchard starring Helen Hayes. As a child she also made regular appearances on a local children's television series, The Merry Mailman, hosted by Ray Heatherton.
Allen returned to Broadway for a 1955 revival of Finian's Rainbow. She was in the cast of the original off-Broadway production of Hair at Joseph Papp's Public Theater and also appeared in George M! before receiving critical acclaim and a Tony Award nomination for Two Gentlemen of Verona, which earned her New York Drama Critics' Circle, Drama Desk, Theatre World, and Outer Critics Circle Awards for her performance. Despite her success, it proved to be her last Broadway appearance to date.
Allen's film credits include Cotton Comes to Harlem, The Hotel New Hampshire, and The River Niger, for which she won an NAACP Image Award. She had a regular role in the dramatic series Dr. Quinn, Medicine Woman from 1993-97. Other television appearances include Barney Miller, The Love Boat, All in the Family, Trapper John, M.D., Hill Street Blues, Cagney and Lacey, ER, and Girlfriends. She played a lesbian prison inmate in the 1975 television movie Cage Without a Key, which starred Susan Dey.
Her most notable roles are the flamboyant and outspoken Doreen Jackson on the NBC soap opera, Generations and Lucinda Cavender, the vampire witch in the horror comedy film The Midnight Hour. Before her role of Doreen on Generations, Jonelle played ambitious salesgirl turned boutique manager Stacey Russell, on the short-lived primetime soap, Berrenger's.
Allen appeared as the legendary Harlem Jazz Queen Florence Mills in Harlem Renaissance at the 2007 Edinburgh Festival Fringe.
She is currently working at the Conservatory of the Arts in San Juan Capistrano.