The 19-year-old earned the Cobblers all three points from the game, as United lost a league game for the first time since December 15.
Johnson was sent off after the goal, his celebrations earned him a second yellow, but he remains positive for next season.
"The adrenaline was pumping and I ran to celebrate with the fans, but rules are rules and it was a second booking," he said.
"Hopefully next season we can move up and forward, and keep improving.
"If we can keep playing how we played on Monday I am sure we will secure a top-half finish and that will prove a good season for us."