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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.