"[2][7] After acting on 36 TV programs in two years as a freelance, in 1951, Sinclair signed a long-term contract with CBS, becoming the first person to join what an article in The New York Times termed "video's incubator for hatching its own stars.

But not long after signing with CBS, she played quite different parts on three successive evenings: a vicious singer, a spiteful flapper and a libidinous shrew.

[3] And she starred in productions of Wuthering Heights, The Scarlet Letter and Little Women;[citation needed] also on the Sherlock Holmes television series with British actor, Ronald Howard.

[citation needed] The one major motion picture that Mary Sinclair acted in was Arrowhead made in 1953, starring Charlton Heston, Brian Keith and Katy Jurado, with Jack Palance as an Apache chief, in which she played Lela Wilson.

[3] In the 1960s, as her television career faded, although attending the Actors Studio in Manhattan, headed by Lee Strasberg,[10] and appearing on the stage, Sinclair, in the main, retired from acting, and devoted most of her creative energies to painting.