With World Cup hero Jonny Wilkinson currently battling to overcome a persistent shoulder/neck problem, Grayson showed he is ready to step in if required for Six Nations duty.
He kicked 21 points as Northampton wiped out a 15-10 interval deficit to triumph 36-15 and move third in the table.
"Paul has a very astute tactical brain. He has applied himself magnificently over the last two years," said Smith.
Grayson booted five penalties and converted all three Northampton tries by lock Rob Hunter, wing Bruce Reihana and scrum-half Mark Robinson as Northampton underlined their play-off credentials.
Hunter was carried off with his neck in a brace after claiming the opening touchdown and was taken to hospital during the half-time break as a precaution.
His second-row partner Jon Phillips limped off after just seven minutes with a suspected calf muscle injury and is not thought likely to be fit for next weekend's Heineken Cup clash against the Borders.
London Irish have now suffered successive home defeats against fellow play-off contenders Northampton and Wasps, and the players can expect some straight talking from coach Gary Gold this week.
"I am furious. It was the same as last week against Wasps you just cannot afford to make errors against quality teams, because you know it will cost you," he said.
Irish now find themselves with something of a back-row injury crisis as they prepare for next Sunday's Parker Pen Challenge Cup clash with Narbonne.
Paul Gustard (shoulder), Declan Danaher (leg) and Chris Sheasby (calf) all missed yesterday's game while flanker Kieron Dawson is a long-term absentee.
"As it stands, we don't have back row for next week," added Gold.
"But we hope both Paul Gustard and Declan Danaher will be available."
Irish suffered another injury blow today when their captain Ryan Strudwick was forced off with shoulder trouble, and he is unlikely to feature against Narbonne.
London Irish scorers: Tries: Sackey, Horak. Cons: Mapletoft. Pens: Mapletoft.
LONDON IRISH: Horak, Sackey, Appleford, Hoadley, Bishop, Mapletoft, Edwards, Hatley, Drotske, Durant, Strudwick, Casey, Roche, Haslett, Murphy.
Northampton scorers: Tries: Hunter, Reihana, Robinson. Cons: Grayson 3. Pens: Grayson 5.
NORTHAMPTON: Beal, Reihana, Hyndman, Leslie, Cohen, Grayson, Robinson, Smith, Thompson, Taumepeau, Hunter, Phillips, Connors, Fox, Blowers.