Sir George Mansfield Smith-Cumming (died 1923) was the first director of what would become MI6. In this role he was particularly successful in building a post-imperial intelligence service.

Born into a middle-class family, Smith attended the Royal Naval College at Dartmouth and, upon graduation, was commissioned a sub-lieutenant. He was posted to the HMS Belleraphon in 1878 and for the next seven years saw sea duty in the East Indies. However he increasingly suffered from severe seasickness, and in 1885 was placed on the retired list as "unfit for service ".

He was recalled to duty into the foreign section of Naval Intelligence in 1898 and undertook many missions. He would travel through eastern Germany and the Balkans pretending to be a German businessman, even though he didn't speak any German. His work was so successful that he was recruited to the Secret Service Bureau as the director of the foreign section. During this period he married the extremely rich May Cumming, and as part of the marriage changed his name to Smith-Cumming.

In 1909 Major Vernon Kell became director of the newly-formed Secret Intelligence Bureau (SIB), created to deal with growing public opinion that all Germans living in England were spies. In 1911 the various security organizations were re-organized under the SIS, Kell's division becoming the Home Section, and Cumming's from the SSB becoming new Foreign Section, responsible for all operations outside Britain. Over the next few years he became known as 'C', after his habit of initialing papers he had read with a C written in green ink. This habit stuck for later directors, although now the C refers to "chief".

In 1914 he was involved in a serious road accident in France, his son was killed and he had to have a leg amputated. From then on he often interrupt conversations in his office by suddenly stabbing his new wooden leg with a knife or letter opener, telling all sorts of fantastic stories as to how he lost his leg.

Budgets were severely limited prior to World War I, and Smith-Cumming came to rely heavily on Sidney Reilly, a spy based in St. Petersburg. At the outbreak of war he was able to work with Vernon Kell and Sir Basil Thomson of the Special Branch to arrest twenty-two German spies in England. Eleven were executed, as was Sir Roger Casement, found guilty of treason in 1916. During the war the offices were renamed, the Home Section became MI5 or Secret Service, while Smith-Cumming's Foreign Section became MI6 or the Secret Intelligence Service. Agents who worked for MI6 during the war included John Buchan and Somerset Maugham.

Secret Service budgets were once again severely cut after the end of WWI, and MI6 stations in Madrid, Lisbon, Zurich and Luxembourg were closed.

Sir Mansfield Smith-Cumming died in 1923.

It is believed that Ian Fleming's James Bond character M is based loosely on Smith-Cumming, changing the C used by Smith-Cumming to M in reference to Sir Miles Messervy , a later director of MI6.