Much of the early part of Florian Leopold Gassmann’s life is shrouded in mystery. He was born in Brüx (Most), a small town north-west of Prague, in 1729. Ernst Ludwig Gerber (1790) claimed that Gassmann learned the fundamentals of music at the Jesuit Gymnasium in Komotau and it is also believed that one of his earliest teachers was the regens chori at Brüx, Johann Woborschil, with whom he studied singing, violin and the harp. Gassmann’s father, a goldsmith, was opposed to his son pursuing a musical career and had him apprenticed to a local merchant at the age of twelve. Shortly afterward he ran away, eking out a precarious existence playing the harp in Karlsbad, if one account is to be believed, before making his way to Italy where he may have studied with the celebrated theorist Padre Martini. Gassmann probably served a conventional apprenticeship as a young composer writing church music until such time as he attracted enough interest to be awarded an opera commission. Gassmann’s first opera, Merope, was produced at the Teatro S Moisè, Venice, in the Carnival season of 1757. The work was a success and Gassmann composed a new opera annually for the next five years. His third opera, Gli uccelatori, was the first of his many settings of libretti by Carlo Goldoni.

Gassmann’s setting of Metastasio’s Catone in Utica was staged at the Burgtheater in the 1761- 62 season and the following year he was invited to Vienna as ballet composer and successor to Gluck. Under the terms of his contract Gassmann also agreed to compose operas the first of which was another opera seria by Metastasio, Olimpiade, which was staged at the Burgtheater in October 1764. Symphonies by Gassmann were also performed at court – these may well have been written in Italy in anticipation of his new post in Vienna – and Gassmann also directed performances of other composers' operas. During the year of mourning following the death of Franz I (1765-66) the Viennese theatres were closed and Gassmann returned to Venice where his opera Achille in Sciro was produced at the Teatro S Giovanni Grisostomo. On this trip he met Antonio Salieri and brought him back to Vienna as a pupil. Returning to Vienna in the spring of 1766 Gassmann found a dearth of opera seria singers but an exceptional buffo ensemble and for the next few years he concentrated on writing comic opera. His setting of Goldoni’s L’amore artigiano (produced April 1767) was to achieve the greatest success of all his operas and is a key work in the evolution of Viennese opera buffa. Gassmann also wrote a considerable amount of chamber music during his Viennese years and his quartets were particular favourites of Joseph II. Burney also thought highly of these works writing:

It is but justice to say, that since my return to England, I have had these pieces tried, and have found them excellent: there is pleasing melody, free from caprice and affectation; sound harmony, and the contrivances and imitations are ingenious, without the least confusion. In short, the style is sober and sedate, without dulness; and masterly, without pedantry.

Gassmann was the founder of the oldest Viennese musical society, the Tonkünstler-Sozietät, and his oratorio La Betulia liberata was written for one of the society’s first public performances (19 March 1772). The same month he succeeded the younger Georg Reutter as Hofkapellmeister and immediately began an important reorganisation of the court chapel’s personnel and library. Gassmann’s tenure of the position was brief: he died in 1774 as a result of a fall from a carriage. As a token of the high regard in which he was held by the imperial family the Empress Maria Theresia was godmother to his second daughter, who was born after his death.