Ernst was born on July 28, 1922, in Jaffrey, New Hampshire. She had two younger brothers. She attended school in Europe and according to her, "I'd probably been better off in a school right here at home." Her family did not approve of her becoming an actress, enrolling in a drama school, and joining the Mercury Theater Group. Ernst did multiple jobs including nailing scenery and delivering posters by driving the company's Ford Model T.
Her first role was in the 1939 Broadway production Too Many Girls, directed by George Abbott. Ernst said that she tried to have an audition for Abbott for months, but that Abbott had trouble arranging it. Actor Eddie Bracken said that he would tell Abbott about her after hearing her sing and he later sent a note to Ernst which stated that Abbott did not want to see her, but for her to come anyway. With the help of a stage carpenter who played the piano for her and encouragement from Bracken to run out there, Ernst barged into a dance audition that Abbott was holding to sing and dance for him. Abbott gave Ernst the role and she signed a contract for $40 a week for Too Many Girls.
Two months after her role in Too Many Girls, Paramount Pictures asked her to travel to Hollywood for a screen test for a role in a then untitled film in the Henry Aldrich film series. The screen test cost $1,200 and had a film crew of 24. It had no audio. Ernst and Jackie Cooper were told by the director Theodore Reed to ad lib as if there were no film crew surrounding them. She starred in the 1941 film Life with Henry directed by Reed which was the second film in the Henry Aldrich film series.
|Too Many Girls||1939–1940||Tallulah Lou||Imperial Theatre||October 18, 1939 – February 11, 1940|||
|Pal Joey||1940–1941||Linda English||Ethel Barrymore Theatre||December 25, 1940 – August 16, 1941|||
|Truckline Cafe||1946||Sissie||Belasco Theatre||February 27 – March 9|||
|If The Shoe Fits||1946||Cinderella||New Century Theatre||December 5–21|||
- "Boston Deb". The Boston Globe. February 26, 1940. Retrieved August 28, 2019 – via Newspapers.com.
- "Leila Ernst". Internet Broadway Database. Retrieved August 31, 2019.
- S. Truesdell, John (September 1, 1940). "Former Usher and Debutante". The Cincinnati Enquirer. Retrieved August 28, 2019 – via Newspapers.com.
- Francis, Robert (April 6, 1941). "Leila Ernst Knew What She Wanted; Went and Got It". The Brooklyn Daily Eagle. Retrieved August 28, 2019 – via Newspapers.com.
- Crowther, Bosley (February 6, 1941). "The Screen; Hen-reeeee!". The New York Times. Retrieved September 1, 2019.
- "Death notices". Boston Globe. June 1, 1970. p. 24. Retrieved September 7, 2019 – via Newspapers.com.
Veitch – Of Foxboro, formerly of Croton on Hudson N.Y., Leila (Ernst), residence 159 1/2 Cocasset st., wife of the late James Veitch, sister of Frederick T. Ernst of Jaffrey, N.H. and Richard H. Ernst of Foxboro.
- "Burial records" (PDF). www.townofjaffrey.com. p. 22. Retrieved September 7, 2019.
- "LEILA ERNST A BRIDE; Ingenue of 'Pal Joey' Married Here to Stacy B. Hulse Jr". The New York Times. July 1, 1941. p. 27. Retrieved September 7, 2019.
- "Leila Ernst". Playbill. Retrieved August 28, 2019.