After battling from 15 points down to lead Harlequins 30-24 with the 80 minutes up, Ben Botica barged over for a try and converted to steal a 31-30 win in the LV= Cup.
Mallinder was annoyed to see a second-half comeback wasted and stressed Northampton need to find a suitable response at Newport next Sunday after a fifth loss in six games.
"We need to stick together, go down to Wales and get a good win," said Mallinder. "We are going through a down period and we need to get back to winning ways.
"We are very disappointed to concede that late try. The three first-half tries were stupid really as well. We conceded tries too easily, they were all our mistakes and all preventable."
Sam Smith scored two tries thanks to two wild passes from Ryan Lamb, and an overthrown lineout gifted Harry Sloan a try. A stern talking-to from Mallinder at half-time saw Saints come back with tries from Luther Burrell and Tom May.
"At half-time I said, 'Let's simplify things'," said Mallinder. "Our possession was fantastic from the scrum and lineout, and we said, 'Let's look after the ball, take the simple option and not the impossible one'.
"I am not going to criticise anybody, but there were two tries we didn't have to concede and an overthrow at our lineout.
"They are parts of our game we will look at and need to cut out. Ryan Lamb generally had a very good game but he did make a couple of bad passes."
Further cause for worry for Northampton came when Christian Day limped off with a recurrence of his ankle injury.
Harlequins director of rugby Conor O'Shea saluted the industry in his team after they kept pushing for victory even after slipping behind.
He will have George Lowe back from injury next week, and Lowe is set to take over at centre from Matt Hopper, whose performance today was singled out by O'Shea.
The Quins boss said: "I said to Matt Hopper, 'Can we have just one more 80 minutes and take next week off', because we will have George Lowe back, and he was outstanding for us, as he has been all season.
"The work ethic was fantastic. Even in the second half the work ethic in defence when we were under massive pressure was great.
"We managed to stem the tide and as always happens the momentum came back to us and we took the chance."
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