David Ashford has been made interim home affairs minister following the death of Bill Malarkey.
Mr Malarkey died last week aged 68 after living with cancer for several years.
Mr Ashford, who is the current health minister, will hold both positions until Mr Malarkey's permanent successor is selected.
Under Manx law, all departments must have someone "who has the warrant of a minister" to sign off on decisions.
A government reshuffle will take place following the next Legislative Council election, which is scheduled to take place on 12 March, Chief Miister Howard Quayle said.
The Department of Home Affairs has responsibility for police, the fire and rescue service, the probation service and the communications commission.
Mr Ashford said he was "honoured" to take on the position to continue the "hard work put in" by Mr Malarkey.