boyd was born in largs, ayrshire, to john knox boyd, a bank agent, and his wife margaret wilson smith, the younger boyd attended largs academy before studying medicine at glasgow university under sir robert muir and carl browning. he came top in his year, securing the brunton medal, when he graduated mb chb in 1913 (he subsequently secured the diploma in public health (dph) from cambridge in 1924 and the doctor of medicine (md) degree from glasgow in 1948) and joined the ramc the following year, serving in france, belgium and salonika in the first world war. he remained with the ramc until 1946, also seeing action in the second world war in the middle east and north west europe; for the year 1945–46, he was director of pathology at the war office.