A year following his graduation in 1883, Loomis became a reporter for the New York Tribune and later assumed a campaign press relations position.

It was during the administration of President Benjamin Harrison that Loomis first entered government service as consul at Saint-Étienne, and at Grenoble, France, until 1893.

During his State Department tenure, he became associated with the reorganization of the American Red Cross, serving as a charter member.

His commissions included final negotiations which resulted in the acquisition of the Panama Canal Zone, service as special ambassador to France to receive the body of John Paul Jones and Special Envoy Extraordinary to Japan, arranging the visit of the U.S. fleet to that country in 1908.

Shortly before World War I Loomis returned to private business as foreign trade adviser to the Standard Oil Company serving until retirement.