The Grade II-listed building in the heart of the town was built in 1855 but has been unused for a decade.
After securing £2.4m of Heritage Lottery Fund cash, the council plans to create a library and business hub.
Following an ownership row, the council took it over in July 2016 and has announced work will start in January.
Originally, the building housed assembly rooms and a court.
However, it passed into private ownership about a decade ago and concerns were raised about its deteriorating condition as it fell into disrepair.
After the council failed to find a solution with the owner, the Welsh Government successfully applied for a compulsory purchase order.
Conservation work on the main structure and exterior of the building will now begin in the coming weeks.
Anglesey council's planning portfolio holder Richard Dew called it an "ambitious project" which will preserve "a significant piece of Holyhead's rich history".
A multi-million pound project to transform Holyhead's derelict market hall into a community hub is set to get under way, Anglesey council has said.