Tries from David Ribbans, Rory Hutchinson and Henry Taylor put Saints 21-16 up at the break, with Matt Gallagher grabbing Sarries' only try.

Ben Spencer added to his 11 first-half points with three more penalties before Grayson won it with his late kick.

While both sides were without a host of World Cup stars, Sarries were hardest hit, with a league-high 11 still away.

Prop Titi Lamositele was the only one of the 15 Saracens players involved in Japan to return for the Premiership opener, following the end of the USA's World Cup campaign last week.

Seven of the legion of Sarries players that remain in the Far East starred for England just hours earlier, when Owen Farrell captained the Red Rose to victory over Australia to reach the semi-finals.

Northampton pair Courtney Lawes and Lewis Ludlam were also part of that win, while Saints still had five away on international duty.

Even without some of the game's brightest talents on the pitch, the two makeshift sides produced an exciting spectacle at Allianz Park.

Spencer booted Saracens into the lead inside two minutes, then slotted his second kick before Ribbans marked his 50th appearance with a try to edge Saints ahead for the first time.

Gallagher's try and a further Spencer penalty put the Londoners 16-7 with 10 minutes of the first half remaining.

Hutchinson did well to wriggle through Saracens' defence for Northampton's second score soon after, before Taylor finished a great counter attack to put the visitors five points up at the interval.

Saints had a fourth try ruled out by the television match official on 48 minutes when replays showed that Taqele Naiyaravoro sent George Furbank over with a forward pass.

The bonus-point try never came, but two Grayson penalties proved enough to seal a victory which Spencer had almost secured for his own side, only for a late penalty to decisively come back off the post.

Saracens director of rugby Mark McCall told BBC Radio London:

"It is horrible to lose games like that when it is the last minute.

"We were in a tough situation just after half-time when they had scored two tries, and we found a way to get back into the game and that is very pleasing with a young side.

"We showed a bit of resilience out there and were two minutes away from having a different feeling to what we have got right now. Our effort, attitude and ability to bounce back from set-backs was good."

Northampton Saints director of rugby Chris Boyd:

"It was a young leadership group today so to force the mistake and kick a goal to win the game was pleasing.

"Credit to the boys, they kept their nerve and there's always two ways of looking at a game, but from an unbiased point of view, I think we did enough to deserve the win.

"Grayson's game management was good. He's coming to terms with his personal attitude to the game and what suits the team, so he's finding that sweet spot. Each week he has got better for us but he has a big job to do when Dan Biggar is away and he becomes our alpha male."

Saracens: Gallagher; Segun, Lozowski, Tompkins, Lewington; Goode (capt), Spencer; Adams-Hale, Pifeleti, Lamositele, Skelton, Isiekwe, Clark, Earl, Wray.

Replacements: Gray, Barrington, Wainwright, Kpoku, Hunter-Hill, Whiteley, Manu Vunipola, Obatoyinbo.

Northampton Saints: Furbank; Collins, Dingwell, Hutchinson, Naiyaravoro; Grayson, Taylor; Waller (capt), Fish, Hill, Ribbans, Moon, Coles, Tonks, Wood.

Replacements: Van Vuuren, Van Wyk, Painter, Bean, Eadie, Tupai, Symons, Sleightholme.