The Cherries chairman, who has immersed himself in activity in the local community throughout his time at the club, takes up the role with immediate effect as he will be involved in all aspects of governance for the Trust.
Mostyn visited Pokesdown Community Primary School in January as part of the Goadsby respect campaign and with his new role, the chairman will be influential in leading visits within the community.
He said: "I absolutely love the work that Steve Cuss and his incredible team do in the community and I wanted to be a part of it.
"I had no hesitation at all about the role. Particularly with our Barclays Premier League status, we want to use that to help the community even more and the role will help to bring the Football Club and the Community Sports Trust closer together."
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