Despite good early pressure from Bath Rugby, Northampton opened up a 9-3 lead before Peter Jorgensen added a converted try deep in first half injury time.
The Saints furthered the lead with a second try for Jorgensen before Bath finally hit back with a try from Tom Voyce.
"It was a well below-par performance," said Mike Foley. "We lost the battle in the breakdown and the Northampton Saints defence is the best we have come across in the Premiership."
