Frank Thornton, the third child of William Ernest Ball, a bank manager, and Rosina, whose other children were Marjorie, who tragically passed away in 1980, and John, was born with the surname Thornton, which was his mother's maiden name, and the middle name of his father, William Ernest. As a young man, Frank developed a passion for music, playing the cello in his school's orchestra, and he also served as a corporal in the Officer's Training Corps, leaving in 1937 to pursue a career in finance as a clerk with the Guardian Insurance Company in London.
However, Frank's interests soon shifted towards the performing arts, and he decided to follow in the footsteps of a colleague who had left to become an actor, enrolling in the London School of Dramatic Art's evening classes. In 1939, the school relocated to Whitney in Oxfordshire, and Frank, still a student, began acting in the local repertory company, which featured the talented Peter Jones.
In 1941, Frank made his West End debut with Donald Wolfit, and the following year, he appeared in the Manchester Opera House production of The Scarlet Pimpernel, where he met his future wife, actress Beryl Evans. Frank's career continued to flourish, and in 1943, he joined the Royal Air Force, training as a navigator in Nova Scotia. He became a pilot officer and remained in the entertainment unit after the war, serving alongside fellow corporals Peter Sellers, Dick Emery, and Tony Hancock.
Frank was demobilized in 1947 and returned to the stage, starring in the musical The Dancing Years. He also began to make appearances on television, including a stint as a compere on the variety show The Centre Show, which featured the debut of Hugh Lloyd in mid-November 1950. Frank married Beryl on January 5, 1945, and they had a daughter, Jane, in 1946. The couple settled in West Wickham, Kent, where they lived for many years.