William George MacCallum was born on April 18, 1874, in Dunnville, Ontario, Canada. He was the older of two sons and second of four children of George Alexander MacCallum and Florence Octavia (Eakins) MacCallum, both from small Ontario towns.
His father, the son of a Scottish immigrant, was a physician who maintained a busy country practice and, for a time, served as medical superintendent of two Ontario hospitals for the insane.
Deeply interested in natural history, he cultivated in both his sons (the younger of whom also became a physician) a broad approach to scientific investigation. MacCallum's mother was an accomplished pianist and singer who gave her children a deep love of music.