Mercer served in the
American Civil War as assistant surgeon in the
149th Regiment, Illinois Volunteers. In 1866, after graduating Berkshire Medical College, he relocated to
Omaha, Nebraska, to begin medical practice. He also became secretary of the Nebraska Medical Association, vice-president of the
American Medical Association and was a co-founder of
Omaha Medical College. In 1868, he helped establish Omaha's first hospital, located at what is now Cass and 26th Streets. It later burned down. He formed Omaha Medical and Surgical Institute, which he ran for many years. He abandoned that venture to become chief surgeon of the
Union Pacific Railroad. From the 1880s until his death, Mercer operated Mercer Chemical Company in Omaha's
Old Market district. He also formed, in 1889, Omaha Motor Railway Company, which became a direct competitor of Omaha Street Railway Company. The two later merged. == Personal life ==