Mostyn Hall, near Holywell, which is set in 25 acres (10 hectares) of formal gardens, has been home to the Mostyn family for generations.
Their connection with the area dates back more than 500 years.
The garden is being opened for the first time under the National Garden Scheme which was set up to benefit nursing charities.
The estate has a Japanese garden and a restored Victorian kitchen garden.
The private gardens of an historic house in Flintshire are being opened to the public on Sunday.