The 45-year-old has handed in his notice to the council after four years at the helm.
He is going to be chief operating officer at the Department for the Environment Food and Rural Affairs (DEFRA).
He said: "I have had a great four years in the Royal Borough. It has been a real pleasure and privilege to work with so many passionate and committed people, both inside the council and in our community.
"We have all worked hard together to take the council to new levels of achievement and performance."
He added: "I leave behind a committed and ambitious officer team who I know will continue to do their utmost to ensure that borough residents remain the top priority in every service."
A graduate of the University of London, Mr Trenholm holds an MBA from Durham University Business School.
He started his career with the Royal Hong Kong Police Service and then moved to Surrey Police before working in the private sector.
He worked in Buckinghamshire County Council before taking up his position with the Royal Borough in spring 2008.
Since then he has been in charge of all council employees and responsible for working with councillors on the running of the council.
Council leader David Burbage thanked Mr Trenholm for his part in helping the council deliver ‘more for less’.
"We are very sorry to lose him but wish him well in his new role," he added.
A council spokeswoman said details of Mr Trenholm's departure date and replacement have not been confirmed.
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