Herbert, who joined the Cobblers in September 2009, has made 42 appearances for the club, the majority of which have been as a substitute, scoring twice.
"Courtney is still learning the game," said boss Gary Johnson.
"He has been training hard, but he needs games. He had the opportunity to go to a couple of clubs and he has chosen Cambridge.
"We will keep a regular eye on him and his progression, but this move will give him the chance to play regular football which is what he needs."
Source: PA
Source: PA