The 21-year-old joined the League Two side on a short-term deal at the start of the season, but a string of impressive performances have seen him earn an extension.
D'Ath has scored four goals in 10 games for the Cobblers so far this term, with two of those strikes coming in his last three appearances.
"He has done really well for us, either starting games or coming off the bench," manager Chris Wilder told the club's official site.
"We feel he has done enough, and shown enough potential, for us to want him to be part of the squad we are building for the longer term."
Northampton are currently in ninth place in the League One table after nine games.