He was most likely born in the village of Lauw near Tongeren in the prince-bishopric of Liège (now in the province of Limburg, Belgium), though the date is only known approximately.
During this period Brassart most likely composed the motet O flos fragrans, which was popular enough to appear in several manuscripts of the time, as well as Te dignitas presularis.
Nevertheless, some of Brassart's music has survived, including 11 motets, 8 introits, and many individual mass movements.
All of his surviving music is sacred, and includes mass movements, introits, and numerous motets; one of his pieces is on a German text, and almost certainly was written during his employment with the Imperial chapel.
His most famous motet, O flos fragrans, is modeled on a similar work by Dufay, and the two composers may have known each other well.