She said, "I think this arrangement is well suited to some of the large authorities that want to make significant changes and I think it will be successful."
Under the terms of the deal, the joint venture, called RBT (Rotherham Brought Together) Connect, will eventually manage all of the council's IT functions, procurement, revenues and benefits, human resources and payroll.
Mountain believes this approach could help larger councils deal with difficult technology changes. "In very large authorities, making changes is more complex and external parties with business change experience can help them," she said.
The joint venture was approved at a council meeting last week and negotiations are now taking place to finalise the agreement, which is expected to start in April.
BT will invest £30m in technology as part of the deal, with the remaining £120m coming from the council. Up to 600 council staff will be seconded to BT, although they will remain council employees and their conditions of employment will not change.
Rotherham Council has estimated that the joint venture could generate £50m worth of savings without any significant increase in the council's current IT expenditure.