The star died on 27 April (11) from complications of Alzheimer's disease in Newbury Park, California.
Her career spanned five decades with roles on TV and the stage, and her stint in the 1969 Broadway hit Promises, Promises earned her a Tony statuette and a Drama Desk Award.
Mercer made her Broadway debut in 1969 in Greenwillow, before winning the title role in an off-Broadway production of the musical parody Little Mary Sunshine the following year (70).
She won plenty of praise from critics for her part in the 1978 revival of Stop the World, I Want to Get Off, in which she played Sammy Davis Jr.'s girlfriend.
Her manager and agent Martin Gage has paid tribute to the late star, remembering her spirit and comedic flair
Gage says, "She was lovely, crazy but lovely. She could take a part that had been done before and make it a laugh riot."
Mercer is survived by her husband Patrick Hogan and a daughter, Deirdre Whitaker.