[2] William S. Johnstone was born and raised in Brooklyn, New York City, to a Scottish-born father and a German-born mother.

[5] Johnstone acted on stage with the Theatre Guild at the beginning of his career, appearing in a number of bit parts.

[10] Cummings later commented, "William Johnstone made a marvelously attractive unhero ..."[10][7] He also appeared that year in a lead role in Kate Clugston's These Days.

He had a memorable role in the ".22 Rifle for Christmas" episode of Dragnet on radio, and reprised it on the television series.

In film, he portrayed John Jacob Astor IV in Titanic, and had a supporting role in Down Three Dark Streets.