According to Houbraken, Rembrandt studied with him in his workshop a few months after his six-month apprenticeship with Pieter Lastman, before he opened his own workshop in Leiden.
According to the RKD he was the brother of Jan Pynas who travelled to Italy. Their sister Meynsge married the artist Jan Tengnagel in 1611. Though Jacob is known for scenes of Italy, these paintings could have been based on sketches brought back by his brother Jan and it is not certain that Jacob travelled to Italy. He influenced Rembrandt and became a member of the Delft Guild of St. Luke in the years 1632-1639. He became the teacher of Bartholomeus Breenberg.
The Pynas brothers were grouped within Dutch artists called the Pre-Rembrandtists. Their work is close in style to the painter Adam Elsheimer, and there has been a history of mis-attribution between the three, where the Pynas brothers are known to have signed their works "J. Pynas."
Jan died in Amsterdam; Jacob survived him by many years and is thought to have died in Delft.