Leonard George Thiele was born in Brisbane, to Maude Henrietta Thiele, née Rasmussen, and Herman Albert Thiele, a chemist. He attended Milton State Primary School and Brisbane Grammar School (1934–38) on a scholarship. However, the family's financial situation during the Great Depression forced Leonard to leave school and enter the workforce. He worked as a junior clerk at Brisbane City Council's Electricity Supply Department. In his spare time, he took up amateur drama, with local repertory groups. From the age of 17, he augmented these activities with a role as a part-time radio announcer, after successfully auditioning at the Australian Broadcasting Commission (ABC) in Brisbane.