That night, the 10pm exit polls condemned Mrs May to an uncomfortable niche in history as the Prime Minister who went to the country confident of a resounding victory, but instead blew her own majority.
There was nothing for the Conservatives to celebrate then, and precious little for them to raise a glass to on the anniversary.
The year that has passed since then has only magnified the consequences of the breathtakingly inept Conservative campaign, which future students of politics will view as a masterclass in how not to contest an election.
Remembering Mrs May stubbornly parroting her campaign slogan of “strong and stable” while she appeared anything but as the party’s manifesto unravelled around her, should make anybody whose natural political sympathies lie with the Conservatives shudder.
Every struggle the Government has faced since June 8, last year, and every uncertainty that lies ahead, is linked to the sheer awfulness of that campaign.
The Prime Minister’s image of calm competence, carefully nurtured during her years as Home Secretary, vanished instantly. Worse, so did the authority that had made her the best and most obvious choice to take over from David Cameron.
Suddenly, a politician who appeared to be at the pinnacle of her career was transformed into a vastly diminished figure living on borrowed time.
And that is how Mrs May remains. That she has survived for an entire year is the most notable achievement of her premiership so far, but it has happened only because the party fears electoral disaster if it foists yet another new leader on the country.
Knowing that she remains in office because she is regarded as the least-worst option by her own colleagues – for now – is a miserable position for Mrs May to be in, but she has been the architect of her own misfortune.
Others appear gleeful to capitalise on her precarious position, and not just Labour for whom a badly weakened Prime Minister is a gift to its hopes of regaining power.
Mrs May might take some comfort from recent opinion polls that show those questioned regard her as a better Prime Minister than Jeremy Corbyn would make, but there is little to cheer her from within the party she leads.
Enemies are plentiful, notably amongst the ranks of the hard Brexiteers. It is a measure of Mrs May’s weakness that Jacob Rees-Mogg, a backbencher, appears to wield at least as much influence within the parliamentary party as she does.
Whenever he speaks, Conservatives give him their full attention. He can be openly critical of his party leader without the slightest fear of any sanction, and is able to issue warnings to her with impunity.
Whether one agrees with Mr Rees-Mogg about Brexit or not, this is an extraordinary state of affairs. The Conservatives, the party of discipline and purpose, look instead like rats fighting in a sack.
At least he has the decency to remain on the backbenches, and not seek a ministerial post to use as a platform for his views.
The same cannot be said for those around the Cabinet table who will not hesitate to wield the dagger when they think the time is right, notably Boris Johnson. Every intervention by the Foreign Secretary underlines how weak Mrs May is.
The problem with all this is that it is not just an issue for the Conservatives. It poses a real risk to the entire country.
Friday’s anniversary is not the only milestone likely to cross Mrs May’s mind this week. The other falls on Saturday, which marks 300 days until Britain leaves the EU on March 29, next year.
Is Britain ready to do so, with a clear strategy for the future? No, not at all. So much remains unclear, whether it be customs arrangements, the Northern Ireland border, trade agreements or future levels of immigration, that the country knows hardly more about what life will be like outside the EU than it did on the morning after the referendum.
And all this uncertainty stems from that woefully misjudged election, because Mrs May simply lacks the authority over her own colleagues to establish and enforce clear policies.
Instead, she is like a boxer on the ropes, soaking up punishment and desperately hoping the knockout blow doesn’t land.
This isn’t any way to run a Government, let alone guide the country through the greatest peacetime challenge in a century.
So the process drifts along, risking a botched outcome that could cause serious economic harm to businesses and the people they employ, and all because of political hubris a year ago.
That was worrying enough at the time. But nothing like as troubling as its potential consequences over the coming year.