City have won just once in 10 league outings this term, and are without a home victory all season.
The final straw for Millen came at Bloomfield Road on Saturday, where the Robins were thrashed 5-0 by Blackpool, leaving many City fans calling for the manager to be axed.
And on Monday morning the club announced that they had parted ways with the 45-year-old, ending a 12-year association in various capacities.
In a statement, Robins chairman Colin Sexstone praised Millen's work at Ashton Gate and wished him well for the future.
"I'd like to place on record my thanks to Keith for his 12 years of service at the club," said Sexstone.
"He is an outstanding coach and we wish him all the very best for the future."
Source: PA
Source: PA