The former All Black fly-half was at his creative best as he teased the Falcons defence with his varied passing game, while also posing a constant threat with his turn of pace and eye for the gap.
The win means Saints can finally afford to relax in the Guinness Premiership and look forward to finishing the season on a high, and Grayson was fully aware of the importance of Spencer's contribution.
"He's an exceptional talent," Grayson said. "He is one of the most talented players to have ever picked up a ball and that's a fact.
"We are starting to see some of that, although it is not a surprise to me because I have seen him train every day.
"He is fitting in well to the structures we have got and he is applying the game-plan well.
"Having a solid platform helps so he can direct, and when he attacks a line and runs at defences you have got to keep a very close eye on him. It's a joy when he's on."
Spencer was forced off the field after 68 minutes with a leg injury but Grayson revealed it was nothing serious.
"It's just a bang on the leg. He got trapped on the wrong side of a ruck and there was a pretty long queue to give him a kick I suppose," he joked.
Saints racked up five tries in the comprehensive 32-13 win at Kingston Park, and Falcons director of rugby Rob Andrew was at a loss to explain his side's dire display.
"It was our worst performance of the season," Andrew said.
He refused, however, to blame the loss on the absence of centre Mathew Tait, who was absent on England Sevens duty.
"We know what the rules are and we know what the regulations are. He is a core Sevens player and that is the situation. The system is as it is."