William O’Grady dumped an estimated 29,000 tonnes of construction waste at two sites near Caernarfon in Gwynedd, Mold Crown Court heard.
He had denied 12 charges brought by Environment Agency Wales.
O’Grady will be sentenced later and now faces a proceeds of crime investigation.
The charges were brought against O’Grady and two companies he runs, Gwynedd Skip Hire and W. M O’Grady Haulage and Plant Hire.
Judge Niclas Parry told him: “The financial consequences of your convictions are going to be your punishment.”
The three-week trial heard the construction waste dumped by O’Grady included bricks, plastic, wood, glass, plaster and insulation fibres.
The charges centred on land at Bryn Awelon, Llanfaglan near Caernarfon, in 2007, and at his home at Tyddyn Whisgin, at Caeathro, near Caernarfon, in 2008.
He dumped about 27,500 tonnes at one site, believed to be one of the biggest illegal landfill sites in north Wales, and about 1,500 tonnes of waste at the other.
The prosecution said the land at Bryn Awelon, next to The Foryd Bay nature reserve, was a site of special scientific interest.
O’Grady accepted that there was waste on the site but said neither he nor his companies had put it there.
The court heard the larger volume of waste at the other site was producing “leachate,” a polluting liquid produced by rotting waste, and puddles contained bubbles of escaping gas, believed to be hydrogen sulphide.
O’Grady said that material was not waste at all but a harmless aggregate which was ideal for improving land.
The Environment Agency said the waste was contaminated, and posed a risk to wildlife and people nearby.
After the verdict, Sian Williams, from Environment Agency Wales, said: “It is against the law and simply irresponsible to dump waste wherever and however you like for financial gain.
“The defendant knew all too well that there are rules and laws to follow and he chose to ignore them.
“These laws are there not only to protect the environment, but also to make sure there is a level playing field for other companies that operate legally.”