She was the daughter of actress Ann Codrington (real name Marjorie Doris Codrington, who appeared in films such as The Rossiter Case) and her first husband, Percy Charles Penn-Gaskell, a military.
In December 1915, her mother, while pregnant with Patricia, and her grandmother, Mrs. Helen Codrington, were aboard the British passenger liner SS Persia when it was sunk by a German submarine in the Mediterranean.
Hilliard attended the Royal Academy of Dramatic Art in London, where her performance in Molière's Sicilien and her striking beauty led to a 2-year contract with Warner Brothers.
She rapidly progressed, being the female lead in The Girl in the Crowd (1935) and René Clair's The Ghost Goes West (1935).
She took a break between 1940 and 1944 following the birth of her first child and while her husband was on active military service in World War II, returning to the stage in 1944.