The 39-year-old guided United to promotion via the Sky Bet League One play-offs last season, and was dismissed with the club sitting 20th in the Championship table, a point above the relegation zone.
Despite a tricky run consisting of just one victory in 15 games, the U’s have not once entered the bottom three this season.
READ ALSO: U’s state of play in search for Des Buckingham replacement
READ MORE: Des Buckingham sacking has left a very sour taste in the mouth
Buckingham posted on social media: “Managing my boyhood club was always a dream of mine, and as such, I want to thank the board for giving me an opportunity that was so close to my heart.
“Last season’s promotion created memories that myself, my family and no doubt the fans will never forget.
“I was brought into the club to try to get it to the Championship and hopefully I’ve left it in a better place than when I arrived.
“I would have loved the opportunity to achieve this season’s objective of staying up, but I respect the club’s decision to part ways and go in a different direction.
“I would like to thank the players and staff for their dedication and commitment during my time.
“They’re both a credit to themselves and this football club. I’m immensely proud of what we were able to achieve in such a short period of time.
“Finally, I’d like to thank the fans for their support of both myself and the players, which was vital in getting us into the Championship for the first time in 25 years. I’ll always be one of you. Yellows.”