On-loan Oldham marksman Ricketts converted from the spot in the 84th minute to secure the Saddlers a fortunate success after a low-key display.
Former England international Ricketts had a hand in winning the penalty as it was his pass that released Ishmael Demontagnac, who was hauled down by right-back Danny Jackman.
Ricketts made no mistake from the spot to grab his third goal for Walsall.
But the defeat was rough on Northampton who have now lost four of their last five games.
They failed to make their first-half domination pay and should have broken the deadlock themselves three minutes before Ricketts struck.
Andy Holt had a free-header from just six yards after a cross from Poul Hubertz but headed weakly straight at Clayton Ince.
It proved a costly miss - as did Mark Hughes' in injury time when he steered his header over the bar.
In truth, Northampton should have been well in control by half-time. They dominated the opening period only to be let down by their finishing and a goal-line clearance by Rhys Weston.
Ryan Gilligan twice went close from long range in the opening 15 minutes before Colin Larkin lashed a shot just wide.
Walsall struggled to cope with the physical presence of Hubertz and he was unlucky not to break the deadlock in the final four minutes of the half.
He first headed wide when well placed before seeing his 44th minute header from a Daniel Jones free-kick cleared off the line by the well-placed Weston.
That lucky escape finally sparked Walsall into life and it needed a fingertip save by Mark Bunn in stoppage time to deny Ricketts from 20 yards, but he was powerless to deny the well-travelled marksman from the spot.