After the war, Mason became a mercenary, aiding Chile and Peru during the Chincha Islands War and then briefly joining the Cuban rebellion against Spain in the Ten Years' War. In 1870, along with several other Union and Confederate veterans, he joined the General Staff of the Khedivate of Egypt and conducted explorations in Egypt, Nubia, Darfur and Equatoria. In 1877 he charted Lake Albert and discovered the Semliki River. During the 1870s he traveled extensively with Charles George Gordon and served as Gordon's deputy and chief of staff during Gordon's anti-slavery campaigns. In the 1880s Mason became governor of Massawa prior to the Italian occupation of that town in 1885. Henry Morton Stanley praised the quality of Mason's map of Lake Albert during the Emin Pasha Relief Expedition. Mason represented Egypt in meetings with King Yohannes IV of Abysinnia and was a signatory to the 1884 Hewett Treaty which would eventually lead to the First Italo-Ethiopian War and the famous Battle of Adowa.