Peter Dennis was born in Dorking, Surrey, the son of Michael Henry Dennis, a mechanical engineer, and Violet Frances Lockwood, a housewife. He was one of four children including two brothers, Michael and David, and a sister, Dorothy.
His early education was in a Roman Catholic convent. He continued his early studies in Portobello Road, London at the North Kensington Secondary School, until the age of 14. He spent the next four years training as an accountant and as a surveyor. While employed at T. S. Appleton & Son Ltd. in Shepherd's Bush, he was called up for his national service in the British Army.
After graduating from RADA, he returned to the Birmingham Rep to play numerous leading roles. He went on to perform at the Everyman Theatre, Liverpool, the Liverpool Repertory Theatre, and in London's West End. He also began to be a regular presence on British television, appearing in The Avengers among many other shows.
