A family spokesperson said she died at a Boca Raton hospice after suffering from cancer.
Before and after marrying Nat in 1948, Maria Cole had her own singing career, performing with greats such as Count Basie and Duke Ellington.
After her husband died in 1965, also from cancer, she created the Cole Cancer Foundation.
Born in Boston in 1922, Maria moved to North Carolina as child after her mother died. She later moved to New York to pursue a music career.
Duke Ellington heard recordings of her singing and hired her as a vocalist with his orchestra.
She stayed with him until 1946 when she went solo at the city's Club Zanzibar as an opening act for the Mills Brothers.
It was there she met her future husband, and she continued to travel and perform with Nat throughout the 1950s.
Her children, Natalie, Timolin and Casey Cole, said in a joint statement: "Our mum was in a class all by herself.
"She epitomised class, elegance, and truly defined what it is to be a real lady. She died how she lived - with great strength, courage and dignity, surrounded by her loving family."
Maria Cole, widow of Nat King Cole and mother to singer Natalie Cole, has died in Florida aged 89.