She was the daughter of Imperial Count of the Realm Anton Corfiz Ulfeld, who "held several high political and court appointments" and his second wife Maria Elisabeth, Princess von Lobkowitz. At age 17 she married Count Franz Josef Anton von Thun und Hohenstein, who later became an Imperial Chamberlain. In the 1750s, the young Countess Uhlfeld studied keyboard with imperial court organist Wenzel Raimund Birck, a respected teacher and composer. A manuscript book of simple keyboard pieces and exercises that he prepared for her survives. Whether, as has been suggested, she also studied with Joseph Haydn, is difficult to determine, since the source indicating this only gives the title "Countess Thun;" this name was also held by other women over time. The Countess evidently became a very skilled musician. The visiting English musicologist Charles Burney praised her harpsichord playing in print, saying that she "possesses as great skill in music as any person of distinction I ever knew." The salon that developed in her home is described by Clive as "a focal point of the musical and social life of the Viennese aristocracy." She had six children, of whom four survived into adulthood:
It is possible that Countess Thun first met Mozart in 1762, when she was 18 and he was seven; this was during an early concert tour of the Mozart family, carried out to display their children as musical prodigies; the young Mozart performed in her father's home. In 1781, when the 25-year-old Mozart moved permanently to Vienna to pursue his career, he and Thun became friends. Mozart wrote of her to his father Leopold, " the most charming and lovable lady I have ever met; and I am very high in her favor." He frequently performed in her home, and she lent him her excellent Stein piano when Mozart performed before the Emperor in competition with Muzio Clementi on 24 December 1781. Thun may have played an essential role in Mozart's career when she arranged for him to perform extracts from his recent opera Idomeneo in her home before a set of guests that included Count Orsini-Rosenberg, the manager of the Imperial Theater. The Count "applauded warmly", and not long thereafter gave his agreement to the plans to commission Mozart for the opera Die Entführung aus dem Serail, which turned out to be his first great success in Vienna. As Mozart composed the work, Countess Thun listened with encouragement to each of three acts of the opera, performed on the piano by Mozart in her home, as he completed them. According to Kenyon, "after 1782, features less often in his activities." After Mozart's death in 1791, it is believed that she helped financially with the schooling of his two surviving sons. She was the dedicatee of Beethoven's Piano Trio in B flat, Opus 11.
