McCallum attended the Royal Military College in Woolwich and began his colonial service career in 1874. He was governor of Lagos Colony before coming to Newfoundland in 1899. The friction between McCallum and Prime Minister Robert Bond resulted in his recall in 1901. McCallum then became governor of Natal in 1901 and then governor of Ceylon in 1907.
McCallum retired from colonial service in 1913 and returned to England.