The charity has been looking for premises from which to run a shop since a fire destroyed its Youngtown shop and $500,000 worth of uninsured stock last month.
City Mission executive officer Albert Van Zetten said that if professional indemnity insurance could be found, half of the 5000sqm building in Merino St would be retained as a roller rink with the other half used as a shop.
"That's the preferred option," he said.
"If we can't get the insurance, we'll use half as storage and half as a shop.
"We hope to open on March 1."
A donation from a Launceston couple, who wish to remain anonymous, had made the sale possible, Mr Van Zetten said.
"It was a significant donation," he said.
Gunns has provided the charity with a Killafaddy warehouse rent-free for six months to store donated goods, which Mr Van Zetten said had been flowing at a higher than normal rate after the fire.
"The public support has been incredible," he said.
"I thank the people of Launceston for that."
The sale contract is expected to be signed today or tomorrow and the business will close on February 16.
Mr Van Zetten said the Youngtown site could still be rebuilt, depending on the size of the insurance payout.