He was the commandant of the Medical Department Professional Service Schools in Washington, D. C. (now known as the Walter Reed Army Institute of Research), founding commandant of the Walter Reed Tropical Medicine Course.

Afterwards he served in the Hawaii Department 1916–1918 and as a pathologist at Army Medical Center in 1939.

[dubious – discuss] Callender was awarded the Richard Pearson Strong Palladium Medal by the American Foundation for Tropical Medicine in 1946.

Gen. Callender's supervision, guidance and research in the school contributed directly to the achievement of the lowest sick rate of any army in the history of war.

His initiative and effort were largely responsible for the creation of the course in tropical medicine and for the development of extensive research activities at the Army Medical School in 1941 before Pearl Harbor.