Tredegar House, one of the most historic stately homes in south Wales, is to be leased to the National Trust for 50 years.
Newport council took the decision to hand over its running on Tuesday.
The park and children's play area will still be free to visitors, bookings for weddings and other events will be honoured, and tour charges will stay.
The council will keep ownership of the Grade I listed building as lease negotiations continue.
It is hoped a higher profile as the National Trust's first stately home in south Wales will boost paying visitor numbers from 25,000 to 100,000 a year.
Tredegar House was the ancestral home of the Morgan family, later Lords Tredegar, from the 15th Century until 1951.
The building was then used as a private school until the then Newport Corporation bought the property in 1974.
Although the local authority has carried out restoration work in subsequent years, a recent survey found that the house and other buildings on site still needed around £4.5m of major work.
While responsibility for the renovation work is still under negotiation, Newport council leader Matthew Evans said the lease would increase Tredegar House's popularity.
"The National Trust will be able to give it the profile and promotion that it needs and deserves," he said.
"It is a major kudos for Newport and Tredegar House that we were able to attract such a prestigious partner."
Staff, trade unions, the Friends of Tredegar House and other organisations were consulted during the negotiations.
Anne Pegington, chairwoman of the Friends of Tredegar House, said that the society's initial concerns over the plan had been allayed and that members had a "positive outlook".
Once contract details are finalised, it is expected that the National Trust will take over the property in spring 2012.