Buist joins the agency as client services director from digital agency Profero, where he held the same position.
At EHS, he will be responsible for account management across the London and Cirencester offices, working on key accounts such as Barclays, Compare the Market and Diageo.
Corston joins as business development director from the co-creation and brand strategy consultancy, Sense Worldwide, where he was managing director. Previously, he was sales and marketing director at Agency.com and head of digital at BBDO/Craik Jones.
Both Corston and Buist will report to new chief executive Tash Whitmey. Whitmey became CEO of the agency in June following the departure of Matt Atkinson, who left to take up a global role as group marketing and digital officer at Tesco.
She was previously group managing director at the agency.
Whitmey said: "We expect Nick and Andy will enhance the EHS 4D proposition, which embeds direct, digital and data into strategic communications plans, allowing our clients to deliver experiences that connect them with their consumers at all touchpoints."