Former Master Builders Queensland chief executive Grant Galvin said he was "lucky to be here" after years of intimidation and harassment by the CFMEU.
"It was like boiling a frog - the temperature kept going up and I didn't realise how bad it had become until it was almost too late," he told the Queensland Commission of Inquiry into the CFMEU on Tuesday.
"I had a mental breakdown and I attempted to take my own life."
He said a 2017 CFMEU‑led rally of up to 2000 people outside Master Builders' Brisbane headquarters was a key flashpoint, turning into a "mob" that taunted him and hurled sexist abuse and lewd gestures at staff.
The crowd marched on the Spring Hill office chanting "Wake up, Grant! Wake up" to the tune of the Wiggles' children's song Wake Up Jeff, making it clear he was the target.
Warned only the day before by police, Mr Galvin said he was shocked by the rally's scale, organisation and "antagonistic" tone as protesters fanned out around the building, pressing up against glass and staring in at staff across several floors.
In a video played to the inquiry, he said a man sat up in a tree with a coffin directly outside his office window - an image he likened to the grim reaper.
He said the coffin, chants and abuse "crossed a line from protest into something much darker and more personal".
Mr Galvin said some union supporters used the rally to intimidate his colleagues.
About 50 people were working in his office, more than half of them women.
"I heard disgraceful, sexist, swearing, derogatory comments … sexual in nature and directed at women," he said, adding protesters made crude finger gestures towards staff.
Police, who were present before and during the rally, eventually asked workers to move away from windows and into the middle of the building.
"In hindsight, I should have done more about ensuring that those staff were protected. Nobody should have to put up with that behaviour at work," he said.
During the rally, senior CFMEU figure Michael Ravbar, union official Jade Ingham and an Electrical Trades Union representative were escorted inside by police and demanded Mr Galvin support the Building Employees Redundancy Trust.
He said he told them he already backed the scheme and had long supported its work, including fundraising for a suicide‑prevention program linked to the trust, but claimed the union later used the rally to portray his stance as a victory.
In a separate incident at the CFMEU's Bowen Hills office, Mr Galvin said his car was plastered with "strike if provoked" stickers bearing the union's snake logo as officials, including Mr Ravbar and Mr Ingham, laughed outside.
"As far as I'm concerned, it's vandalism, it's intimidation, it's bullying," he said.
"I've worked with pretty much every union known to mankind … I've never seen anything like the CFMEU and the levels that they would go to intimidate and to get what they wanted."
During cross-examination, counsel Ruth O'Gorman suggested CFMEU boss Mr Ravbar had apologised to Mr Galvin after the incident.
Mr Galvin firmly rejected that suggestion, stating: "Knowing Michael over the last 10 years and all our interactions, I don't think he's said sorry to anyone."
The inquiry is examining allegations of criminality and misconduct involving the CFMEU in Queensland's construction industry.
The union denies wrongdoing and the commission has yet to make any findings.
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