The 18-year-old midfielder came on as a second-half substitute before Paul Anderson's goal earned Dale a point.
McWilliams graduated from the club's academy and signed a two-year professional deal last summer.
Asked if it was a dream come true, McWilliams told BBC Radio Northampton: "Yes. This is what I aimed to do."
Last year McWilliams joined Southern League Premier side Kings Lynn on loan, and he believes it was a good move for his development.
"I've been recalled and it's helped my game; I think it's improved my game and I'm ready to show it here," he added.
Justin Edinburgh's Northampton side are 15th in League One, eight points clear of the relegation zone with five games to play.
Teenager Shaun McWilliams says he has fulfilled a dream in making his league debut for hometown club Northampton in the 1-1 draw at Rochdale on Saturday.