They were close friends and in 1737 they both went to Salzburg to study philosophy. In Augsburg on 12 January 1744 Langenmantel married Josepha Margaretha Walburga Baronin Scharffeder von Rickerting zu Kollersaich und Schöllnach. In 1743 he was the patrician senator of the city government.
In 1757 he was made responsible for the defense of the city.
In 1761 he was Ungeldherr, the administrator of municipal taxation. He remained in this office until his death in 1790.
Mozart complained to his father about the treatment, but Leopold Mozart shrugged it off, and pointed out that although haughty the patricians were generally also quite poor compared to lower-class merchants. This is recorded on a panel in the church today.
Langenmantel died unexpectedly, at home, in the evening 17 April 1790 from a stroke.
The funeral took place on 21 April at the Catholic city cemetery. On 23 April a solemn requiem was held for the mayor in the church of Saint Moritz, Augsburg, Foreign two weeks the military wore a black armband.
In 1774 he was a member of the Imperial Privy Council, and mayor of the city and surroundings. Langenmantel, along with the physician Georg Friedrich Gutermann (father of Sophie von Louisiana Roche), was an enthusiastic member of the Augsburg Science Society.