Percy was born Percy Clapperton on 17 May 1897 and christened at Croxley Green. His mother Eliza Emma Clapperton was single, a domestic servant and aged 25 when he was born. Eliza subsequently married a gardener, George Mercer, in Compton, Guildford on 30 December 1905.
By 1901, at the age of 3, Percy was living at Woodwalk, Red Heath, in Watford Civil Parish and Croxley Green Ecclesiastical Parish as the adopted son of John and Emily (nee Ryder) Gifford. According to the census returns, Percy was born at Woodwalk, Croxley Green. John Gifford came originally from Dorset and Emily from Maple Cross. John worked as a carter on a farm. By 1911 he was living as Percy Gifford with the family at 2 New House Cottages, New House Farm in what is now Farm Road, Chorleywood. Percy entered Christ Church School on 10 May 1904. In 1911 he was still at school at the age of 13 but working as a milk boy.
He is recorded as enlisting in Aylesbury, Buckinghamshire.