Before World War I Turnbull worked at the family firm of ship-owners. During the war he served as a driver, and during the Battle of the Somme won the Military Cross. In 1919 he had his first major tennis tournament, the Davis Cup. In 1921 he played again in the Cup and won the singles at the Portuguese Championships, but then disappeared from tennis for four years to focus on golf. In 1926 he returned to the Davis Cup, and in 1928 again won the singles at the Portuguese Championships.