He made the commitment that his party will fight the 2021 Holyrood election “committed to signing no new private finance deals”.
Private finance initiatives were brought in to the UK by the Tory government in the 1990s - but use of the funding model was expanded after Tony Blair came to power.
Mr Leonard said it was “unjust” that private companies are “raking in miilions in profits” under schemes such as PFI, public-private partnerships and the Scottish Government’s NPD system of funding.
Announcing the policy at the Scottish Labour conference in Dundee, he said more funding was needed for public services, and suggested: “Why don’t we simply call a halt to the racking up of extortionate PFI debts which push local services to the brink?”
The left-winger, who took over as leader of Scottish Labour in November, also pledged his party would bring in legislation to cap the profits private agencies make from supplying staff to the NHS.
Labour will offer voters the “biggest programme of social and economic reform in the history of the Scottish Parliament”.
Mr Leonard backed UK party leader Jeremy Corbyn on Brexit, saying the UK “must have a new customs union with our European neighbours”.
While he said the “shambolic Tory party” was to blame for Brexit, he insisted the result of the referendum to leave the European Union must be respected.
But Mr Leonard was also clear: “If the Brexit deal is not right, then Labour will vote down the deal.”
He hit out at the SNP’s stewardship of Scotland’s economy, saying it had “stagnated and stalled” under the nationalists, with growth three times lower than the rest of the UK.
He added: “The SNP is complacent. They tell us the foundations of Scotland’s economy remain strong and short-time, fixed-term contact working is what people want.”
He told how his brother-in-law, Thomas Sim, from Glasgow, had been paid off three times last year and was “scrimping and saving to make ends meet”, driving a taxi when without a job to support his family.
“His life is all too typical of what work is like for too many people in Tory Britain and SNP Scotland,” Mr Leonard said.
“That’s why I am absolutely determined to bring about real change to end this insecurity in the labour market.”
To protect tenants in the private rented sector, he announced Labour would bring a member’s bill to Holyrood to introduce rent controls.
He said this Rent Restrictions Act would be a “Mary Barbour law”, named after the main organiser of rent strikes in the Govan area of Glasgow in 1915.
Having a home is a “basic human right”, he said, as he also pledged a Labour government in Scotland would build more council houses.
Mr Leonard said: “Building more public homes and tackling rip-off rents and agency fees will create better homes for bringing up our children.
“It will reduce the cost pressure of housing and it will provide a supply of sheltered housing too, which will be in growing demand from our ageing population.”