Cambridge have been promoted from Sky Bet League Two after a 3-0 victory over bottom side Grimsby.
United needed a draw to secure promotion, and goals from Liam O’Neil, Wes Hoolahan and Paul Mullin sealed all three points to ensure Cambridge will play in the third tier for the first time since 2002.
After a nervous start to the game, O’Neil struck the key opener after 25 minutes.
Luke Hannant’s free-kick from the left was nodded back across goal by Joe Ironside to O’Neil, who powered home a volley from just outside the six-yard box.
Just before the hour mark Hoolahan, 12 days before his 39th birthday, took a free-kick from wide on the right flank which evaded everybody and found its way into the far corner.
Mullin capped the scoring with a free-kick 10 minutes from the end, netting his 32rd league goal of the campaign to break the record for most goals in a season since the division was rebranded League Two in 2004.