Rest in peace, William Schallert.

The actor — who played the late Patty Duke's onscreen father on The Patty Duke Show — sadly died from unknown causes on Sunday, May 8, his son confirmed to The New York Times.

william schallert getty images

William on The Patty Duke Show in 1963.

Throughout his illustrious career, William had lead roles on shows The Many Loves of Dobie Gillis and Perry Mason. He additionally appeared on TV series Leave It To Beaver, Gunsmoke, The Nancy Walker Show, The Waltons, The New Gidget, The Torkelsons and Desperate Housewives, among others!

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William's wife of 66 years, Rosemarie Diann Waggner, passed away last year. He is survived by his four sons, Edwin, Joseph, Mark and Brendan, and seven grandchildren. Our thoughts are with William's family during this difficult time.