In 1898, having served in minor diplomatic posts, he was appointed Counsel-General in Tangier by President William McKinley and was later made Ambassador.
Gummeré, a personal friend and former business partner of Perdicaris, initially advocated military action, but President Theodore Roosevelt and Secretary of State John Hay were more cautious, arguing that the situation in Morocco — which was maintaining a precarious independence due to the intrigues of foreign powers such as France, Britain, Germany, and Spain — made military action impractical.
Gummeré met with the Ambassadors of France and Britain, and they agreed to help pressure the Sultan to meet Raisuli's demands.
Later, he served as one of the American mediators, under Ambassador to Italy Henry White, in the Algeciras Conference of 1906, which helped resolve European tensions over Morocco.
While the Europeans backed this plan, Moroccan Sultan Abdelaziz was initially reluctant, finally agreeing to sign it on June 18.