Birmingham Bears needed 10 runs from the final over after mounting a stunning comeback.
But Buck took the wicket of Colin de Grandhomme with the Bears requiring just two from two balls before only giving Aaron Thomason a single to ensure Northants at least left with a share of the spoils.
It was the first point of the Vitality Blast campaign for the Steelbacks, who had lost their first five games in the competition.
But they would have hoped to have left Birmingham with a win after racking up their record T20 score of 231 for five, in which Levi lashed an unbeaten 95.
It wasn't to be, and they needed Buck, who finished with figures of three for 38, to hold his nerve late in the day after Ian Bell had blasted an incredible 131 from just 62 balls.
"Ian Bell played an unbelievable knock," Levi said. "He played it very well and kept us under the pressure and big up to him.
"To almost chase down 230-odd is a great effort but also big credit to Nathan Buck for the last over.
"There are not many times a bowler defends 10 in the last over and to do it on a very good wicket with a short boundary and one of the biggest hitters in the world at the other end in De Grandhomme, he has done very, very well there.
"The boundary boards were peppered all game and it was very entertaining, though some of the boys were joking afterwards that it should have just carried on raining and we would have got the point anyway."
Bears star Bell was happy with his own performance, but disappointed his team couldn't quite finish the job.
"I feel like I have been playing well this year and you are always striving to get it spot on out there in the middle when it really matters and I felt that most things I tried tonight came off," Bell said.
"It was a shame that we didn't quite get over the line but at the halfway point we knew we were chasing 231 and it was a tough ask.
"As a batting unit we have scored 200-plus in successive games so that gives us a bit of confidence.
"We have given a lot of people a lot of entertainment today, both Northamptonshire and ourselves, and hopefully there were a lot of youngsters watching who will look at that and want to come back in the future and play for Warwickshire and the Birmingham Bears.
"That's part of T20 but it was a shame for us not to get the win in the end."