The County needed their highest ever fourth innngs score to beat Surrey in first-class cricket, and they got home at 322 for eight to win a see-saw game by two wickets.
Teenager James Sales, on his first-class debut, hit the match-clinching runs after Keogh, Luke Procter (57) and Emilio Gay (50) had worked their team into a winning position.
Keogh batted superbly to reach 99 before being dismissed, and he admitted: "It's the first time I've got in the 90s actually and it's sort of bittersweet.
"If you'd have offered me 99 and we win the game I'm snapping your hand off, but to experience it and get 99, so close to another 100 that could have turned out to win the game, was tough to take.
"But watching the guys go in and the way Luke Procter gritted it out once I got out and Rosso (Adam Rossington) came in and took it to them a bit and then Junior (James Sales) coming in and hitting the winning runs was nice to see."
It was a strong and patient batting performance from Northants, who took 99.1 overs to reach their target.
“In run chases you always need to try and get off to a good start and try and see off the new ball and Emilio (Gay) did that brilliantly," sad Keogh.
"He looked solid and potentially got an interesting decision, but they go your way, sometimes they don't.
"And then I think Luke Procter batted 160 odd balls there for his 50, just blunting the attack, making the bowlers work hard in the heat.
"Partnerships are crucial and it helped me be able to score quite freely to be honest."
Surrey head coach Vikram Solanki was pleased with his team's performance, and felt they ay have deserved more from the game.
“We’re very disappointed to come out on the wrong side," said the former England batsman.
"We had a plan this morning to make sure we were disciplined in the way we went about things. I thought we did that.
"You see the example of Jordan Clark. To run in as he did on an unresponsive pitch, to bowl the number of overs he did, and to bowl as he did, was outstanding.
“I think we got done by the pitch to some extent, and that’s not taking anything away from Northamptonshire because they played very well to chase down 320 in the fourth innings of a game.
"It’s pretty impressive. So, all credit to them. We gave a good account of ourselves today; we just didn’t have enough unfortunately.”