In the next episode of the ABC’s Australian Story, Mr McHarg says his high-profile crusade against the former prime minister led to "a lot of discomfort" within the company he helped create.
He also admits he can see how his climate-change message "might not find favour" with some property industry clients.
The resignation, less than two weeks before last year’s November 24 federal election, forced Colliers International Australia chief executive John Kenny and Victorian head John Marasco to reject claims they encouraged Mr McHarg to quit the successful agency.
Mr McHarg spent almost $200,000 of his own money on full-page ads in The Sydney Morning Herald and The Daily Telegraph in the weeks before the election, urging Bennelong residents to vote "Planet First, Howard Last".
His ads depicted a cartoon of Mr Howard playing a fiddle with the words, "For 11 years, Mr Howard has fiddled. Now Australia burns."
The war was fuelled at a Business Council of Australia meeting more than a year earlier, when Mr Howard said he did not agree with Mr McHarg’s "starry-eyed view of the Stern report (on climate change)" after Mr McHarg questioned Mr Howard’s long-term view of climate change.
The episode airs on Monday.