The council last summer announced plans to spend almost Â£800,000 on series of projects to kick-start the regeneration of the rundown Castlegate area, which included money for essential repairs to the derelict building.
The Friends of the Old Town Hall last month asked why much-needed work to the roof had yet to begin so many months down the line, preventing rain and snow from further damaging the already crumbling structure.
The council responded by saying it had sought to gain access from the owner G1 London Properties but, having had no luck, was now taking legal steps.
Councillor Mazher Iqbal, cabinet member for business and investment, said: "The council has set aside funds to inspect and carry out urgent repairs to prevent rainwater getting in and unauthorised access, but the owner and their agents are claiming they are unable or unwilling to provide a key for the building.
"Therefore a court order is now being sought to allow us to enter the building and carry out the inspection with a view to ordering urgent repairs and fitting new locks."
A spokesman for Colloco, the agent for the property, said it did not have the keys to the building and had passed on the council's request to the owner.
The Star has attempted to contact G1 London Properties but has yet to receive a response.
The friends group said it was now more than a decade since the Victorian Society had placed the building on its list of the nation's most endangered 19th century buildings, and the building had been allowed to decay considerably since then.
"We were all pleased when the council announced in July that it had at last found the cash to pay for urgent repairs to make the building weatherproof - the owners having not done this," it wrote in the February newsletter.
"But at the time of writing – mid-February - the work hasn’t been started and another winter's weather has got inside.
"What on earth is going on? Is it so very difficult to get up on a roof and fix it?
"We hear that some serious wrangling has been going on inside the council about how to approach the task.
"Without going in to detail, and while we would never argue for unnecessary risks to be taken, it seems ludicrous that nothing has happened.
"Latest news indicates even more delay as no-one seems to be able to find a key to the building, not even the owners... you couldn’t make it up."
The Grade II-listed building, which was home to the city's administrative HQ and courts, is still listed for sale by Colloco, but it is understood the owner is now putting together its own plans for the site's regeneration.
The friends group, meanwhile, is seeking to buy the building for the people of Sheffield and restore it at an estimated cost of well over Â£10 million.
It is considering inviting people to buy community shares to help fund the purchase.