The acting Police and Crime Commissioner, Ray Bisby has taken an oath of office following his appointment by the Police and Crime Panel.
The news comes after previous incumbent Jason Ablewhite resigned from his post as commissioner on November 11, after an investigation was launched into an allegation made against him.
Mr Ablewhite, who was elected to the post of commissioner in May 2016, has been referred to the Independent Office for Police Conduct (IOPC) by the chief executive of the office of the police and crime commissioner.
It is understood that an internal investigation was launched after a complaint regarding "a series of messages" with a member of the public on social media was received and was then referred to the IOPC.
Mr Bisby told BBC Cambridgeshire that he was "shocked" following the news.
He said: "The day that Jason resigned was a shock for everybody but from my previous jobs I was in the situation of 'the work has to go on'.
"Having had the experience I have had of filling in for Jason at meetings, talking with partners, talking with the public at surgeries, talking to councils... it was only natural in my thoughts to do the stewardship between now and May."
Members of the crime panel agreed to appoint Mr Bisby to the role at an extraordinary meeting on November 27 following the resignation of Mr Ablewhite.
The taking of the oath was overseen by justice of the peace, Benjamyn Damazer.
As acting commissioner, Mr Bisby took an oath of impartiality which sets out publicly his commitment to tackling the role with integrity whilst recognising the importance of the operational independence of the police service.
Mr Bisby takes over the role until the forthcoming commissioner elections in May 2020.
Mr Bisby said: "I see my role over the next five months as one of good stewardship, continuing to be the voice of the public and progressing the aims of the police and crime plan.
"It is an honour to take on the role and I look forward to continuing to work with Cambridgeshire Constabulary and partners to make our communities as safe as possible."
Mr Bisby started his career in the military before joining the Royal Ulster Constabulary, serving 19 years in Northern Ireland. He moved to Peterborough in 2007 and became actively involved in voluntary work in the community including becoming chairman of the local policing board.