Robert Blair "Paddy" Mayne was born in Newtownards, County Down, the sixth of seven children in a Protestant family. The Maynes were prominent landowners who owned several retail businesses in the town. He was named Robert Blair after a second cousin, who at the time of his birth was a British Army officer serving in the First World War. The family home, Mount Pleasant, is situated on the hills above Newtownards. A paternal ancestor was Gordon Turnbull, who led the famous Scotland Forever Charge at Waterloo.