Evans put Bradford ahead after 12 minutes in Saturday's League Two clash at Valley Parade with a 30-yard shot after controlling a headed pass from Adam Bolder and turning sharply to give keeper Jason Steele no chance.
There was another blow for Northampton after 37 minutes when defender Dean Beckwith was sent off after pulling back Leon Osborne and denying the Bradford player a goal scoring opportunity.
Northampton made light of their handicap, however, and Billy McKay came close to scoring on the stroke of half-time when he saw his shot rebound off the post.
The visitors caused more anxious moments in the Bradford defence before Evans put the result beyond doubt.
The young striker scored his second goal nine minutes from the end when he volleyed the ball into the net from Bolder's cross.