Philip Euen Mitchell was born on 1 May 1890 in London to a Scottish family. His father, Captain Hugh Mitchell (1849–1937) had served in the Royal Engineers, and after retiring had studied law at the Inner Temple and had become a barrister. His father had played for the Royal Engineers team in the 1872 FA Cup Final. His mother, Mary Catherine née Creswell, died when he was two years old, and his father moved to Gibraltar where he built up his legal practice, living at Campamento in Spain. Philip was educated by a French tutor, becoming equally fluent in English, French and Spanish. He won a scholarship to St Paul's School, London.