Saints fly-half Ryan Lamb handed the hosts an early lead, before opposite number Toby Flood led an immediate fightback that resulted in pair of Tigers tries in as many minutes.
"It was painful," Mallinder told ESPN. "We made a good start to the game and did what we talked about.
"We put them under pressure, got six ahead, but then the wheels came off - two tries from them within a couple of minutes.
"It's been a fight to get into the top four and it's going to be right until the end. What we need to do is make sure we win next week."
Northampton travel to Exeter to take on the Chiefs next weekend.