During the Second World War, Clark lived with her sister at the home of their grandparents in Abercanaid, near Merthyr Tydfil, in a small stone house with no electricity, no running water, and a toilet at the bottom of the garden. Her grandfather was a coal miner. Her first ever live audience was at the Colliers' Arms in Abercanaid, near Merthyr Tydfil. She also recalls living just outside London during the blitz, and watching the dogfights in the air and running to air-raid shelters with her sister. Later, when she was eight, she joined other children to record messages with the BBC to be broadcast to their family members in the forces. The recording event was in the Criterion Theatre, an underground theatre that was safe. When the air-raid siren went off, other children were upset and a call went out for someone to step forward and sing to calm them. Petula volunteered, and they liked her voice so much, in the control room they recorded her. Her song was "Mighty Rose".