Saracens director of rugby Mark McCall claimed Northampton's failure to cash in on a two-man advantage was the key to his side's 29-15 win.
Sarries scored 13 unanswered points while numbers down after prop Carlos Nieto was sent to the sin-bin for killing the ball at a ruck and fly-half Owen Farrell joined him when hitting Northampton's Shane Geraghty with a late tackle.
"They had the opportunity to kick the ball in the corner when it is 10-3 but, when he (Stephen Myler) missed that penalty, it gave us a lot of encouragement," said McCall.
"It was a crucial part of the match because 17-3 would have been a lot taller order. I'm delighted with everyone and how we stepped up to it. We have a squad who like each other and, when you are down to 13 men, you have to play for each other which is what happened.
"Having been down to 13 men against a team like Northampton and not conceded is massive. Our defence has been good in the last couple of weeks and was outstanding. This is a tough place to come and so we are just glad we have got a result."
With Sarries two-men down, Saints captain Phil Dowson elected to kick a penalty instead of trying a line-out and drive on the Saracens line but watched in frustration as fly-half Stephen Myler's shot went sailing wide of the posts.
"It is always a tricky one. You want to get points on the board but you could go for the corner and the scrum but, if you don't get the try, you think you should have gone for the kick," said Saints director of rugby Jim Mallinder.
"It was not nice to lose and especially at home. We came second best and I thought Saracens were accurate.
"We were 10-3 up and, at that point, things looked pretty good. That period just at the end of the first-half and start of the second were vital. We had opportunities to score when they were down to 14 men.
"When teams are down to 14 men, we have to learn to look after the ball and not panic. At the end of the first-half, we could have just kept the ball and put the pressure on them. We came back in the second half and nearly got the bonus point but the game was over by then."