The news was announced in a statement to the London Stock Exchange late on Friday afternoon which revealed the club’s legal director Michael Nicholson will step in as acting CEO.
McKay’s appointment by Celtic as successor to long-serving chief executive Peter Lawwell was announced in January.
The 42-year-old, who had worked for the Scottish Rugby Union since 2008 as director of communications and then chief operating officer, joined Celtic in April to shadow Lawwell before officially taking up the CEO role and a place on the board on July 1.
McKay was involved in the appointment of Ange Postecoglou as the new Celtic manager after efforts to recruit former Bournemouth boss Eddie Howe had foundered.
The departure of McKay leaves Celtic to begin the search for another CEO, while the club are also still without a director of football.
“I’d like to thank the board and everyone at Celtic for their support and wish everyone at Celtic every success for the future,” said McKay in a single-line contribution to the official statement issued by the club.
Chairman Ian Bankier expressed his confidence in the abilities of 45-year-old Nicholson to step into the breach. A former sports lawyer, Nicholson joined Celtic in 2013 as company secretary and head of legal before being promoted to director of legal and football affairs in 2019.
In their statement, Celtic say Nicholson “has been at the centre of executive decision making at Celtic during his time at the club, working directly with the board, the chief executive and the football department.
“Michael carries the confidence of the board and his colleagues. He is widely known and highly regarded in football circles in the UK and in Europe.”
Nicholson is hailed as a ‘team player’ by Bankier who stressed Celtic’s determination to enjoy positive momentum under Postecoglou’s management.
Bankier added: “It has been my pleasure to work closely with Michael for more than eight years now. Having been at the heart of the senior executive and reporting to the board, he has a solid grip on how to achieve our immediate and future objectives.
"Michael is an absolute team player and he will lead an experienced and talented team of executives and colleagues at Celtic.
“Everyone at Celtic will of course work collectively and focus strongly on ensuring we continue to drive the club forward in every area, on and off the field.
“The board, Ange, our management team and all our staff will continue to work together with the ultimate objective, as always, of delivering success for our supporters.
“We thank Dom for his efforts on behalf of the club and everyone wishes Dom and his family very well for the future”.