Warren Pickering told reporters on Tuesday he took speed, ecstasy and marijuana before joining the Australian Defence Force at the age of 28.
The now-43-year-old also said he lost his licence for what he called "low level" drink driving at 18 and again at 20 years old.
"The mistakes I made in my 20s hurt people," he said.
"They hurt me. They hurt people I loved and I will live with that forever.
"I've never made the suggestion that I'm any type of angel. So to assume that I would be is an absolute mistake."
His comments come after Mr Pickering's former partner of almost 20 years, Jo Scott, made a series of claims to The Australian newspaper.
She said the pair had used marijuana and amphetamines, including the drug ice, in the home they shared with their daughter and her three sons from a previous marriage.
"(The allegations) are extremely hurtful and I just hope the public can see through them," Mr Pickering told reporters.
He denied ever using methamphetamines.
Mr Pickering also denied he was a drug addict during the press conference, held a little more than one month after being announced as Victorian president of One Nation.
One Nation federal leader Pauline Hanson previously said she was comfortable with Mr Pickering leading the Victorian branch of the party, despite the claims.
"These are allegations that have been put to Warren Pickering and he's denied those allegations," she said.
"I will not be saying any more about it. It is a personal situation."
Mr Pickering is standing for the lower house seat of Pakenham in the November state election.
He previously ran in the 2022 state election and for the federal Senate in 2025 without success.
He had worked in construction and served in the Australian Army as a combat engineer and also worked in Queensland's mining industry before returning to Victoria.
Nationally, primary support for One Nation stood at 30 per cent, ahead of Labor on 29 per cent, according to a Newspoll published in late August.
But the populist party is not polling as well in Victoria.
The state coalition was the frontrunner with 31 per cent of the primary vote, according to a separate Newspoll, ahead of the incumbent Labor on 28 per cent and One Nation on 19 per cent.
The coalition needs a net gain of 16 seats to form a majority government ahead of Labor.
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