The Yorkshire-born leader of the Scottish Labour Party was among the crowd at Huddersfield Town's Leeds Road ground on August 21, 1971 as his beloved Leeds United played Wolves in the first game of the season.
At the end of the previous season Leeds fans invaded the pitch following a controversial home defeat to West Brom which cost them the 1970-71 league title, prompting a ban from playing the opening games of the following season at home.
It remains a vivid memory and Mr Leonard wrote last week: "No matter it was a goalless draw, the scale of the occasion took my breath away."
It's not the kind of anecdote one might expect to hear from a politician north of the border, but prior to moving to Scotland to attend university in 1980 Mr Leonard's outlook was shaped by his experiences growing up in Yorkshire.
His parents were originally from Leeds but moved to Malton in the late 1950s after his father went out to work at a factory in the market town.
Mr Leonard, who retains a distinct Yorkshire accent despite spending most of his life in Scotland, went to a local primary school and then got a place at grant-aided Pocklington School, which offered sponsored places for children from poorer backgrounds. Two sisters and his aunt remain in North Yorkshire to this day.
Introduced to politics through books like Tony Benn's Arguments for Socialism, which he bought in Woolworths in Malton, he describes himself as being "brought up in a Harold Wilson Labour tradition".
"Going back to those times, there were things that happened like the three day week [where Prime Minister Edward Heath limited commercial consumption of electricity to three consecutive days each week to conserve coal supplies] and all that.
"They were, in a sense, points at which people decided which side they were on, were they on the side of Heath and the Tories or were they on the side of the miners. And certainly, we were on the side of the miners."
The deindustrialisation under Margaret Thatcher had a personal impact on his family, with the factory where his father worked closing down in the early 1980s, forcing him to move several hundred miles south to work in a clothing factory in Suffolk.
By this point Mr Leonard was studying Politics and Economics at the University of Stirling in Scotland, and worked his way up the Labour ranks before eventually being elected as an MSP in 2016, winning the Scottish Labour leadership election the following year.
Being English, he says, is "not really a thing at all" in terms of how he is treated by Scottish politicians. The only time his accent has caused controversy was earlier this year when Dehenna Davison, a recently-elected Conservative MP, saw him in a TV interview and suggested the reason Scottish Labour weren't doing well in the polls was because of his "broad Lancashire accent". "I don't know what was the most offensive", Mr Leonard jokes.
Nevertheless he keeps an eye on how his native county and wider northern England is faring, concluding on the basis of recent events that "the difference in power between London and the North is still apparent".
"Despite the fact the Chancellor [Rishi Sunak] is a Yorkshire MP, I think many people view with interest the fact that the coronavirus job support scheme was offering people 60-odd per cent of their wages. And when it became clear that London was going to be part of the national lockdown the figure went back up to 80 per cent."
In Scotland, the government has significantly more power than politicians in northern English counterparts and has around £40 billion to spend a year.
The devolution settlement - which dates back to the passing of the Scotland Act under Tony Blair in 1998 - means that expenditure on public services in Scotland is higher than the UK average.
But this was the product, says Mr Leonard, of work going back years further where different civic leaders got together to thrash out a clear blueprint between the Scottish Parliament and Westminster.
He says the current circumstances, with Brexit meaning powers currently held in Brussels will return to the UK, make it the perfect time for regional leaders to examine what form decentralisation and modernization in the UK could take
He adds: "I think there is a wider point which has come out in the last few months which is about the imbalance of power which still exists.
"We've had 21 years of devolution. We've had 21 years of the Scottish Parliament. But I think, not only from a Scottish perspective but I think from a northern England perspective, there is still clearly a sense that the UK state is still extremely centralised, and the power in the UK is very centralised around the city of London and around Westminster and Whitehall. And I think that needs to change.
"And actually not only has the experience through COVID exposed some of that, the challenges that have been made by municipal leaders in Yorkshire and people like Dan Jarvis and others as well as Andy Burnham in Manchester, there are people in Yorkshire who have stood up against Whitehall to try to get decent mitigation and decent compensation, for business and working people affected by lockdowns.
"And I just think it has shown up why there needs to be a recasting of power in the UK and there ought to be much greater decentralisation and devolution. Whether that takes the form of stronger local governments, whether that takes the form of Yorkshire, which has got the same population of Scotland, establishing its own assembly, that's for the people of Yorkshire to decide.
"But if we're looking to the medium term into the future there is, in my view, a compelling case to try to move towards a more federal UK and a modernised UK."
In September Mr Leonard faced a challenge to his leadership, which included Leeds MP and shadow Minister Rachel Reeves calling for him to consider his position.
But he said this move was short-lived and said he had spoken to Ms Reeves on the morning of his interview with The Yorkshire Post about Brexit.
He said: "There was a move against me by a couple of members of the House of Lords and four members of the Scottish parliamentary group.
"But it fizzled out, and we move on, and everyone's now focused on the future, and the May 6 [Holyrood] election next year which will be extraordinarily important.
"Not only is at stake, who runs the Scottish Government for the next five years, but also at stake plainly, in light of [SNP leader] Nicola Sturgeon's speech to her conference, is the future of the United Kingdom."