Born at Lemberg, he was the second son of Alexander von Mensdorff-Pouilly, Prince Dietrichstein von Nicolsburg, and Alexandrine, born Countess Dietrichstein-Proskau and Leslie. Entering the diplomatic service at an early age, he was assigned in 1886 to the Paris embassy and in 1889 transferred to London, where with short intervals he was ambassador from 1904 to August 13 1914.
He used his family connections with the English court, derived through the marriage of Count Emmanuel Mensdorff-Pouilly (1777–1862) with Queen Victoria's aunt, Princess Sophia of Saxe-Coburg, his friendship with Edward VII and George V, and his popularity in English aristocratic circles, to establish and secure friendly relations between the Cabinets of Vienna and London. In the critical negotiations before the outbreak of World War I he supported every attempt to avert the danger. During the war he was repeatedly entrusted with missions directed towards the restoration of peace. He met Jan Smuts in Switzerland in December 1917, but these negotiations proved as fruitless as those which he conducted with the representatives of the Triple Entente in the last days of the Habsburg Monarchy. At the end of 1920 he represented the Austrian Republic on the occasion of its reception into the League of Nations.