The club's longest-serving coach has been relieved of his duties with Hannover lying 13th in the Bundesliga at the midway stage of the season having lost eight out of eight games on their travels this season.
"This was not an easy decision to make, but together, we were convinced that we wanted to change something," said the club's director of sport Dirk Dufner.
He, club president Martin Kind and Slomka held talks over the festive period, analysing the first half of the season and discussing the future.
"I would like to pass on my personal thanks to Mirko Slomka, who saved Hannover from relegation and then led us twice into the Europa League," Kind said in a statement.
"These successes will always be linked with his name."
Slomka took charge of Hannover in January 2010 and lifted them away from impending relegation.
In the following season, he led Hannover to the best season in the club's history, only narrowly missing out on a place in the Champions League behind Bayern Munich.
They reached the quarter-finals of the Europa League in 2011/12 and, by finishing seventh in the Bundesliga, they qualified for that competition once again, only narrowly missing out on a third successive European campaign by finishing ninth.
The Lower Saxony club now hope to appoint a successor "in the coming days" to lead them into the second half of the season.