It was a big moment for former Gloucester stand-off Lamb, making his debut off the bench, particularly as he had missed a kick minutes earlier with his side two points ahead then watched in horror as Freddie Burns rifled Gloucester in front with a towering drop-goal.
But starting fly-half Stephen Myler was equally important in the victory, kicking 13 points and cleverly creating his team's opening try for captain Phil Dowson.
Some in the 13,000 crowd were puzzled when rugby director Mallinder replaced Myler with Lamb in the 62nd minute, but he defended his decision.
"The game was always going to be tight, and while folk sometimes talk about making too many changes, we wanted to push on from being seven points ahead," he said.
"It was a straight swap. Myler played very well when he was on, kicked his goals and impressed. But Lamb is also a good player who brings something different to the table.
"He didn't get much chance to show his game, but he has had a really good pre-season and trained well in tandem with Stephen. We have a bigger squad this season and we plan to use them.
"You don't see many Ryan Lamb-style fly-halves in the Premiership. He's an instinctive footballer who can make things happen."
Mallinder was also willing to praise his back row of Dowson, Ben Nutley and Callum Clark, adding: "Dowson was outstanding, his energy and leadership and that cracking try.
"And it's fantastic to see a local lad such as Nutley out there, racking up the tackles, and Clark in the thick of the action.
"I was also impressed with Martin Roberts on his debut. It's not easy when you have six top internationals away on World Cup duty and five new faces making their debuts. That is why I'll take the win gladly while accepting we did make life difficult for ourselves late in the second half."
That was when Gloucester, seemingly fading as Saints opened up a 23-13 lead with 20 minutes remaining, stormed back improbably to lead 24-23 going into the last three minutes.
The fact that they failed to seal off that lead and repeat last season's tight win at Northampton utterly frustrated head coach Bryan Redpath.
"It looked as if Will James had conceded a penalty at the breakdown to give Lamb his penalty chance." Redpath said. "But from where I was sitting, a Northampton player came in from the side first, which sparked Will's reaction and the penalty should have been to us.
"However, I was also frustrated that we were five per cent below our execution in the first half and that we were not switched on when Robinson took the tap for his try.
"But I am delighted by the fact we showed the character to come back after seeing Alex Brown forced off just two minutes after half-time then falling 10 points behind.
"Last season we won here by two points, this time we lose by two. Both were fine games of rugby between teams who want to play rugby. Our youngsters excite me and I am convinced we will build on this."
Two goals from Tim Taylor and a charge-down try from Jonny May saw the visitors 13-10 ahead after 30 minutes. But when Roberts scored by the posts on 33 minutes, Saints began to take an increasing grip.
When Freddie Burns replaced Taylor, he kicked a goal but missed what would have been an equalising conversion after Charlie Sharples swept home on the left.
He made amends with the drop-goal only for Lamb to hold his nerve and pinch victory in the 77th minute.
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