Hjulmand had the unenviable task of taking over from Thomas Tuchel last summer following his shock resignation after five seasons in charge.
And he struggled to adjust as Mainz were eliminated from the Europa League and the German Cup at early stages.
He became the sixth managerial casualty??in Bundesliga this season in the wake of the club???s 4-2 defeat to Borussia Dortmund, a result which left them just a point above the relegation zone.
“This relegation struggle now changes the criteria which has led to a change in coach,” Mainz general manager Christian Heidel said.
???Hjulmand is a football expert who without doubt can develop teams, but the club have new priorities in the relegation battle.”
Martin Schmidt has been named as Hjulmand???s successor after a successful spell in charge of the U23 team.
Swiss-born Schmidt joined Mainz from FC Thun in 2010 and guided the youth team to promotion to the German third division last season.
“For our sporting survival, we need along with tactical discipline great passion, fighting spirit, aggressiveness and consistence,” Heidel added.
???Schmidt can give the team the stimulus it has been lacking in recent weeks and months.”