Grayson stepped up with three minutes to go, slotting a superb penalty from close to the halfway line to ensure his side would walk away with a bonus-point 32-31 win.
It was the icing on the cake for the fly-half, who had come into the team and performed heroically throughout the game at cinch Stadium at Franklin's Gardens.
The victory ensured Saints would finish the weekend one point clear of Gloucester in the race for fourth place in the Gallagher Premiership.
And when asked how he handled the heat of the moment when he stepped up to take the late penalty, Grayson said: “I just try to think about the process.
“I missed a couple when I came on against Bath and thankfully that didn't cost us.
“I just try to stick to the process and know that if I get all the things I think about right more often than not the ball will go somewhere near where I'm aiming.
“I'm thankful it went over.
“You don't get many of those, but you practice for them.
“I'm just happy it went over and we got over the line.
"I don't practice too many long distance kicks but you put pressure on kicking in training and you have to make a certain amount before you go in - if you miss you don’t go in.
“It's about trying to replicate that pressure in training and it's just about trying to stick to the process.”
It was the second time a Grayson kick had earned Saints a vital win against Harlequins as he was also the hero when the two sides met at the Twickenham Stoop back in April 2019.
That 20-19 victory was key for Saints as they went on to finish fourth in Chris Boyd's first season in charge.
And now, in what is Boyd's final campaign as Saints boss, they will be hoping to book a play-off place once again.
“I'm massively pleased and in the past two weeks this group has shown a huge amount of fight and what is means to play for each other and play for the fans,” Grayson said.
“We'll keep going until the final whistle and in the past couple of weeks we've pulled it out the bag.
“The momentum keeps rolling on.
“It was massively important to win last Friday and we're in a good position.
“We've got to go to Sarries and we're at home to Newcastle, and fingers crossed we can play a couple more games, in the semi and final.
“We'll have a couple of weeks without games which could potentially help us refresh and then we can go again and fly into it.”