The company, which operates Scenic Luxury Cruises and Tours and Emerald Cruises, said Downing had “decided to leave the business” after five years at the helm.
Downing was named Scenic’s UK general manager in 2017 after Chris Townson left the company.
He had previously held senior management positions at Riviera Travel, The Late Rooms Group, Merlin Entertainments, Legoland and MyTravel.
In a statement, Scenic said it was “actively recruiting” for a general manager.
Scenic thanked Downing for his “robust leadership” over the past few years and wished him “every success in his next venture”.
It comes after UK sales director Joseph Grimley announced his departure from the business last month.
Grimley, who joined the touring and river cruise specialist in 2018, relocated to Northumberland last year and said he had decided to leave the travel industry to support his wife’s digital marketing business.
Scenic Group has started its search for a new UK sales director as the company prepares for his departure.