The Scottish architect had previously stated that his aim was to make SMC the world's biggest architectural firm.
However, McColl's departure is not entirely out of the blue, as he had been demoted from his role of chief executive earlier in the year, with former non-executive chairman Rodney Walker stepping into the breach.
It is the second piece of bad news for the firm in a month, following SMC's failure to head up this year's AJ100, being pipped to the post by perennial chart topper BDP.
McColl's departure followed last week's review of SMC's trading, when a statement to the Stock Exchange said that while some practices are doing well, others are struggling. Walker said he was keen to get profits back on track, which could mean the closure of certain practices.
Board member Gordon Watson has also stepped down from his position, but will continue to working in a business-development role.
In a statement, the firm said: 'The review provided a series of recommendations to improve the profitability of the group, including the way the group approaches the winning and execution of projects, the management structure, resourcing and the cost base of some business units.
'The board has approved decisive action by agreeing to implement the recommendations of the review immediately.'