He is only one of three equestrians (with Mark Todd and Michael Jung) to win two consecutive Olympic titles in the individual three-day event.

Van Ufford became his mentor and enlisted him to the riding school of the Dutch cavalry in Amersfoort, where de Mortanges later worked as instructor between July 1925 and October 1927.

Besides the Olympics, between 1924 and 1936 de Mortanges took part in various international competitions and acted as a riding instructor with the Dutch cavalry.

He escaped in Summer 1943 during a train transfer, and, after traveling through the occupied Belgium and France to Gibraltar, in February 1944 was flown to England.

On 6 July 1946 he became president of the Dutch Olympic Committee, and remained in this position until 7 April 1961, with a break between 1951 and 1958 when he acted as a vice-president.

On 16 December 1920 de Mortanges married Irma Clara baroness Snouckaert van Schauburg.