The deal will see Heritage, which joined the Centerplate Group in January, cater for special events such as weddings within the house and grounds.
It will also provide catering to the attraction's Courtyard tearoom and visitor centre restaurant.
A statement noted that Mount Stuart grows its own produce within its walled gardens and works closely with local producers on the Isle of Bute.
The deal expands the catering company's operations within Scottish heritage properties, with the firm having served the Palace of Holyrood since 2002.
Heritage, which last year was granted a Royal Warrant to supply catering services to the Queen, serves the Scottish National Portrait Gallery, the Scottish National Gallery of Modern Art, The Signet Library and the Royal College of Surgeons in Edinburgh, as well as Hopetoun House on the outskirts of the city and Musselburgh Racecourse.
Executive director Ian D'Annunzio Green said: "Over the last decade we have built a reputation for working within heritage properties as an exclusive partner delivering exceptional dinners, events and weddings, which has seen many of our long-term relationships flourish.
"This major contract represents another superb property in which we will be making our mark, and Heritage Portfolio relishes the opportunity to work with the team at Mount Stuart, one of Scotland's most remarkable properties."