Tomasini was born in Pesaro, Italy, in 1741. Discovered by Paul II Anton, Prince Esterházy, who was interested in music, he was engaged in 1757 as a servant; he was given musical training, and became a violinist in the prince's orchestra at his court in Eisenstadt.
Joseph Haydn became the director of the prince's orchestra in 1761, and Tomasini became konzertmeister. The orchestra continued under the prince's successor, his brother Nikolaus I, from 1762. Haydn wrote violin concerti for Tomasini.
Nikolaus died in 1790 and was succeeded by his son, Prince Anton, who was less interested in music. Tomasini received a pension and was given a chance to tour. Nikolaus II succeeded his father Anton in 1794 and revived the orchestra; in 1802 Tomasini became director of chamber music of Nikolaus II.
Tomasini married Josepha Vogl, and they had twelve children; four of them became court musicians at Esterházy.