Career
He was a city council member from 1576 and became mayor of Leiden five times. According to Houbraken, Swanenburg was a very good painter, whose paintings hung in various council buildings of Leiden. Houbraken was surprised that Swanenburg was never mentioned in Karel van Mander"s Schilder-boeck.
Houbraken was however mistaken, because Swanenburg is mentioned as one of more than 20 disciples of Frans Floris in Van Mander"s long list of pupils.
According to the Netherlands Institute for Art History, Swanenburg was in Antwerp studying with Floris for 6–8 years in his youth, and settled in Leiden after his training in 1565. He briefly fled to Hamburg for a year during the Siege of Leiden, but had returned by 1574.
He kept a large and respected workshop in Leiden, for oiland glass painting. He was buried in the Pieterskerk, Leiden.
Swanenburg"s paintings of various notable burgers of Leiden have survived, as well as a series of monumental historical allegories on the wool trade, an important source of income for the city during his time as councilman.
Swanenburg"s oldest son, Jacob Isaacsz Swanenburg, became a painter and a master of the young Rembrandt. Claes Isaacsz Swanenburg also became a painter and lived and worked in the Hague, where his art was appreciated. The youngest son, Willem Isaacsz Swanenburg, became an engraver.