Mayer was a Baltimore-born son of German immigrants who studied law and was widely traveled in his youth (China and India, Europe, and Mexico). A founding member of the Maryland Historical Society in 1844, Mayer later served as its president (1867-71) and sparked the creation of the invaluable Archives of Maryland series. Mayer edited the life story of Theophilus Conneau (Captain Canot), a slave trader of French and Italian stock who lived on the east coast of Africa in the 1830s and ‘40s. Conneau found himself in Baltimore in 1853 where he met James Hall (a supporter of the Maryland settlement for freed blacks at Cape Palmas, in modern-day Liberia), who thought that Conneau’s story would aid the anti-slavery effort. Hall introduced the reformed slaver to Mayer. Captain Canot was very popular, selling 17,000 copies with re-printings in London and Paris.