Career
As a boy, Janson joined the Knights of Malta and then the army of the Kingdom of France (as was the custom). But he left the army in his late teens and went to study letters. He was appointed titular bishop of Filadelfia in 1655 and was forced to leave the Knights of Malta.
In 1679 he became Bishop of Beauvais.
Thereafter he was appointed Ambassador Extraordinary of France to Poland and then to the Netherlands. Janson announced his support for the Gallican proposals at the 1682 Assembly in Paris.
As a result, Pope Innocent XI refused to elevate Janson to the cardinalate. Innocent XI died seven years later in 1689.
In February 1690, the newly elected Pope Alexander VIII elevated Janson to cardinal and appointed him cardinal-priest of the church of Sant"Agnese fuori le mura in July of that year.
A number of cardinals still resented the position Janson had taken at the Paris assembly, but the Pope insisted Janson had been forgiven for past errors.