[2] In 1408 he was appointed to a prebend of Saint Lambert's Cathedral, Liège, and in 1411 to a canonry in the Royal Church of St Mary in Aachen.
A career ecclesiastic, although only in minor orders, he amassed a number of clerical livings, including a canonry of the Church of St. Denis in Liège.
When John of Walenrode, Prince-Bishop of Liège, died in 1419, de Sart acquired his private library.
The new duke, Philip the Good, relieved de Sart not only of the regency but also of the chancellorship, reinstating his rival Joannes Bont.
In 1442 he was present in the entourage of John of Heinsberg at the coronation in Aachen of Frederick III as King of the Romans.