Prosky was a two-time Tony nominee for Glengarry Glen Ross and A Walk in the Woods; he also appeared on Broadway in Democracy, Twelve Angry Men, and A View from the Bridge. He was a company regular at Washington D.C.'s Arena Stage for over 20 years, and appeared in numerous regional theater productions. Prosky was well known for his role as Sgt. Stan Jablonski in Hill Street Blues, and his additional film and television credits included The Practice, Veronica's Closet, Dead Man Walking, and Mrs. Doubtfire.
He is survived by his sons Andy Prosky and John Prosky, who appeared with him in a revival of The Price earlier this year in Washington, D.C. and Philadelphia, and another son, Stefan Prosky.