The position was vacant following the news that current managing director Dennis Hogan was returning to his native USA to become chief executive of Foodbuy, a Compass subsidiary.
Following his promotion, Garside, currently the managing director of ESS and Chartwells, will report to new deputy chief executive, Dominic Blakemore, who is also starting his new role on 1 October.
Richard Taylor will fill the position of Chartwells managing director, moving up from his current position of chief operating officer of Compass's education arm.
Blakemore said: “I’d like to thank Dennis for the exceptional contribution he’s made to our UK & Ireland business over the last four years.
“His focus on driving sustainable growth has put the business on a strong footing and I’m delighted that Chris will be leading our UK & Ireland team in the next stage of its journey.”
Before his move to Foodbuy's North Carolina headquarters, Compass has said Hogan will remain in the UK & Ireland business until the end of the year and will work with Garside to enable a "smooth transition."
Compass Group set up food and beverage procurement company Foodbuy in 2015 following its acquisition of Acquire Services Ltd.