Police were forced to cone off the entire London Orbital when Geoffrey Hibbert scaled an overhead sign and unfurled banners in protest of his treatment by the family courts on August 15 last year.
He stayed there for eight hours at junction 14 near Hillingdon, West London, causing tailbacks stretching up to 55 miles.
The property developer could face up to seven years in prison.
Adjourning sentencing until next month, Judge Alan Greenwood, said: " I leave you in no doubt whatsoever that this was a very serious offence indeed.
"I want to make it absolutely clear that this court does not regard the right to protest as the right to trample all over other people's rights.
"It certainly does not regard it as a right to seriously endanger other people which is what you did."
During a four-day trial at Harrow Crown Court, jurors watched TV footage of the protest which showed Hibbert dressed as a comic book hero with banners which showed the photograph of his nine-year-old daughter Chyann.
Giving evidence, he said: "I was trying to get the lawmakers to open their eyes. I was trying to do something for many other fathers and mothers who do not get to see their children. I was never in danger and I never put other people in danger."
Highways Agency staff and police took the decision to close all 12 lanes of M25 motorway during rush hour, which also caused severe delays on the M4.
Prosecutor Firona Horlick said: "No doubt there were people going to work, perhaps there were people going to Heathrow or Gatwick who missed their holidays, maybe members of the emergency services were stuck, maybe there were cars full of children.Mr Hibbert put himself above all of these people."
The conviction came as a pair men dressed as a Jester and The Incredible Hulk staged a protest on top of an Oxford tower, claiming to represent a Fathers 4 Justice splinter group, New Fathers 4 Justice.