Count (from 1858 Prince) Moriz Joseph Johann von Dietrichstein-Proskau-Leslie (19 February 1775 in Vienna; 27 August 1864 in Vienna) was the son of Karl Johann von Dietrichstein -Proskau-Leslie, the 7th Imperial Prince of Dietrichstein (1728–1808), and brother of Franz Joseph von Dietrichstein. He entered the Austrian military service in 1791, became adjutant to General Mack in Naples in 1798 , where he and his superior were taken prisoner by the French, and again in Ulm in 1805. In 1815, he became tutor to the Duke of Reichstadt (until 1831).
Appointed Imperial and Royal Chamberlain as early as 1796, he served as Court Music Director from 1819 to 1826 and as Director of the Court Theatre from 1821 to 1826. In 1826, he assumed the directorship of the Imperial Court Library. He donated Haydn's autograph manuscript of the hymn "Gott erhalte Franz den Kaiser" (God Save Emperor Franz) to the library in 1829 and acquired the score of Mozart's Requiem for it in 1838.
In 1845 he became Lord Chamberlain and retired in 1848. Like his brother, Prince Franz Joseph, he was averse to the Metternich system. He greatly supported Beethoven. As director of the Coin and Antiquities Cabinet (1833–48), he earned further significant merit. He composed songs, dances, and minuets. In 1834, he was appointed an honorary member of the Bavarian Academy of Sciences. After the death of his nephew, Joseph Franz von Dietrichstein-Proskau-Leslie, 9th Prince of Dietrichstein (1798–1858), Moritz Joseph Johann succeeded him in 1858 as the 10th Prince of Dietrichstein and holder of the Lordship of Nikolsburg in Moravia.
Moriz von Dietrichstein was buried in the Hietzing Cemetery, Group 6, No. 15. With his death, the Dietrichstein family line became extinct in the male line. Four years later, the Austrian statesman and Foreign Minister Alexander Count von Mensdorff-Pouilly (1813–1871), who had been married since 1857 to a daughter of the 9th Prince of Dietrichstein, was elevated to the Austrian princely rank by Emperor Franz Joseph under the name Prince of Dietrichstein zu Nikolsburg and inherited the Dietrichstein family estate.