Sir David, who owns the St Mary’s Isle Estate, takes over from Simon Craufurd of Craufurdland Estate who held the role for the past two years.
Sir David will work with SLE’s chairman Mark Tennant and chief executive Sarah-Jane Laing as well as local colleagues and members to ensure the needs of farmers, rural businesses, estates and communities across Dumfries and Galloway are considered by government.
He will also help steer SLE’s work in the south west of Scotland.
Sir David has 55 years practical experience of land management, but has decided to retire this year.
A chartered surveyor by profession, and following a short stint in the city where he set up a merchant bank, he returned to Kirkcudbright to the family estate and farms.
In the 1960s the St Mary's Isle Estate centred on a much more agriculture business, milking 550 cows by 1970 and was one of the largest herds in Scotland in one ownership. Since then the estate has reduced the intensity of its farming.
Sir David has a strong interest in renewable energy and the estate now has wind turbines, solar panels, as well as private rented housing and moderate forestry.
Mr Tennant said: “I am delighted to have Sir David join the SLE team at such an important time.
"His extensive land management experience, passion for wildlife and the environment and first-hand understanding of the key issues impacting rural businesses, will help us ensure rural Scotland thrives.
"I would like to thank Simon for his hard work and dedication, as he steps down after two years in the chair.”
Sir David added: “After enjoying more than 55 years of hands-on management of the farms and other aspects of St Mary’s Isle Estate, I am looking forward to retiring.
"Land managers play a hugely important role in tackling climate change and my new position at SLE will enable me to use my knowledge and experience to help other land managers, rural businesses and communities, especially across Dumfries and Galloway.