Rory Kleinveldt smashed 71 not out, reaching 50 off just 23 balls in the evening sun as Northants reached 268-6, 167 runs ahead.
Adam Rossington's 58 was the hosts' other main contribution of the day.
Earlier, Ben Sanderson, Kleinveldt and new recruit Nathan Buck all bowled tightly as wickets tumbled, with only Aneurin Donald (34) passing 20.
Glamorgan's decision to bat first was a brave one under cloudy skies, and their batsmen never showed the necessary application against an accurate Northants seam attack, supported by smart catching.
Klienveldt and Buck both took 3-35 while Sanderson matched their three-wicket haul with figures of 3-20 to ensure Glamorgan were all out in just 31.3 overs.
Later, the visitors' newly-signed South African paceman Marchant de Lange trapped England batsman Ben Duckett lbw for 12 as Northants lost three early wickets, but recovered well as batting conditions eased in the final session.
Kleinveldt then launched into the Glamorgan bowlers, striking eight fours and four sixes in a brutal display of power-hitting worthy of a T20 match.
Northamptonshire all-rounder Rory Kleinveldt told BBC Radio Northampton:
"I wanted to bowl first and thankfully we did. We were relentless with the ball and put them under massive pressure and kept them under the pump.
"The guys worked hard with the bat early on to set up the situation for me and Crooky coming in. I tried to back myself to clear the ropes and use the short boundary.
"From here, if we can bowl as well as we have done today in the second innings, we should have no trouble in winning this game."
Glamorgan captain Jacques Rudolph told BBC Wales Sport:
"We were a bit in between batting and bowling first, they've left some grass on it but we felt that underneath the surface was quite good, and to be honest we got it wrong.
"There was more lateral movement than we accounted for, but their captain was unsure whether he would bat or bowl first.
"We've got to try and find ways back into this game. Once the wicket flattens out, and the ball becomes older, it becomes a lot easier to bat so we have to try to go big when we get our opportunity again.
"This pitch has got the potential for one of our batters to get a double hundred and get us back into the game."
Northants controlled day one against Glamorgan after bowling the visitors out for just 101 at the County Ground.