The Touched by an Angel star will play the mother of homicide detective Gloria Sheppard (Ally Walker), according to TV Line.
The character will make her first appearance in the season's seventh episode, expected to air in late July.
Duke first rose to fame as a child star, winning an Academy Award aged 15 for her performance in 1962 film The Miracle Worker.
She subsequently starred in ABC sitcom The Patty Duke Show from 1963 to 1966, and later appeared in such films as 1967's Valley of the Dolls, 1978's The Swarm and 1992's Prelude to a Kiss.
Her past television credits include guest roles on Family Law, Frasier and Judging Amy.
The Protector premiered on Lifetime on Sunday night, attracting just 1.9m viewers, and continues this week at 10/9c.
> More US Television news