Before entering government, Ambassador McCallum was a partner with the Atlanta, Georgia, law firm of Alston & Bird, specializing in trial and appellate practice. During his twenty-eight years with that firm, he was appointed by the Attorney General of the State of Georgia as a Special Assistant Attorney General, and he was the recipient of the Atlanta Bar Association Leadership Award in 2001. Ambassador McCallum has authored numerous legal articles and publications and also served as a lecturer on a variety of legal topics.

Over his career, Ambassador McCallum has been active in numerous civic organizations, including service as a Director of the YMCA Blue Ridge Assembly in Black Mountain, North Carolina; President of the Yale Club of Georgia; State and District Secretary for the Rhodes Scholarship Selection Committees; Georgia Representative of the Yale Law School Alumni/ae Association; and as Vice Chairman of the Board of Trustees of the Choate Rosemary Hall School in Wallingford, Connecticut.

Ambassador McCallum was born in Memphis, Tennessee, in 1946. He attended Yale University as an undergraduate on a National Merit Scholarship, receiving his B.A., cum laude, in History in 1968. After graduation, he attended Christ Church at Oxford University on a Rhodes Scholarship, earning a B.A. in Jurisprudence with First Class Honors in 1971. He obtained his J.D. from Yale Law School on an NCAA Post-Graduate Scholarship in 1973 and entered the practice of law that same year.

An avid sportsman in his youth, Ambassador McCallum was captain of the Yale tennis team, the point guard on the Yale basketball team, and a member of the University sides in both at Oxford. He continues a keen interest in sports of all kinds, but now only as a spectator. In addition to spectator sports, his interests include hiking, fishing, the visual arts, architecture, and the theater.