She will become an independent non-executive director of the global biopharma company with effect from 1 October.
Mackay is to leave GSK at the end of 2023 and become an independent non-executive director of Schroders next year.
Dietz is a former president of the American Society of Human Genetics and founder of biopharmaceutical company Blade Therapeutics.
Beal will succeed Lynn Elsenhans as chair of GSK’s corporate responsibility committee following the 2022 AGM.
Rick Haythornthwaite is to step down as Centrica chairman, while senior finance chiefs are set to leave GSK and National Grid.