Fielding Johnson was born in Leicester, the son of Thomas Fielding Johnson Jr. (1856–1931) and Florence Lyne Paget (1856–1933). He was educated at Rugby School, and was a cadet in the school's Junior Division of the Officers' Training Corps. He was commissioned as a Second Lieutenant in the Leicestershire ("Prince Albert's Own") Yeomanry on 16 March 1913, a mounted infantry unit of the Territorial Force.
