Daniel Nardiello sparked the Millers` recovery after their recent disappointing home form with a goal from the penalty spot five minutes before the break.
But two goals in three minutes set the seal on Rotherham's performance. Ben Pringle doubled Rotherham's advantage after 75 minutes and David Noble made sure the points were secured minutes later.
Clarke Carlisle's injury time header for Northampton was no more than a consolation.
Evans said: "It was an important win for us. We said it was it was important we matched the physicality of the most direct team in English football, and they have some quality when they do it.
"They are very good, but we stood up to their missile approach and we managed their hustle and bustle and managed to get the ball down and play. I think we were unfortunate it was 3-1 today because it could have been much more, but we would have taken 1-0 or 29-28.
"It was important we had the physicality because that is not one of our strong points. Northampton are the form team of the division and that's no surprise because they have some good players, but we proved when we can get the players out we want to get out we can win football matches.
"At no stage did I feel uncomfortable because we were passing it properly. I said to them at half time be even more brave because we always believe if we can get at teams and pass the ball we can open teams up and it the second half we got two more goals and were unlucky we didn't get four or five."