But his curmudgeonly image belies a charisma that helped him win a surge in support in this year's presidential election, finishing fourth in the vote that brought Emmanuel Macron to power.
Melenchon went on to win a seat in the national assembly in June, leading the 17 MPs from his France Unbowed party into parliament with raised fists and shouts of "Resistance!"
Since then, despite leading only the fifth-biggest party in parliament, he has carved out a position as the main voice of opposition to Macron's pro-business agenda, blasting his labour reforms as a "social welfare coup
"We represent the alternative to the world that you represent," he told Prime Minister Edouard Philippe after hearing the new government set out its policy plans.
The mainstream Socialists and rightwing Republicans are both leaderless and adrift after Macron's centrists won a landslide - leaving Melenchon with a platform he has wasted no time in using.
The high-tax, high-spend leftist is set to lead a major anti-government protest Saturday, calling on supporters to "lash" Paris.
He has accused "Macron the pharaoh" of monarchical leanings, describing the ex-investment banker - with horror - as Britain's New Labour prime minister Tony Blair and arch-conservative predecessor Margaret Thatcher "rolled into one".
'Power to the people'
Born in Morocco, Melenchon studied philosophy and was a Trotskyist student activist before joining the Socialists at 25 and becoming the youngest member of the Senate in 1986.
He served as vocational education minister under Socialist premier Lionel Jospin from 2000 to 2002.
But in 2008, Melenchon fell out with party leader Francois Hollande and quit the Socialists, saying "our country needs another voice on the left".
The divorced father-of-one launched his first presidential campaign four years later, claiming a modest 11 percent at the head of the Left Party.
But he boosted that to 19 percent this year under his new France Unbowed brand, tapping into widespread disillusionment with the political class and putting in strong debate performances.
He likes to stress that he was "just 600,000" votes away from qualifying for the run-off round of the presidential election - which could have seen him go on to win the presidency.
A passionate orator known for speaking without notes, Melenchon took far-right leader Marine Le Pen to task throughout the campaign, challenging her hard line on immigration.
And he ran a tech-savvy campaign, projecting holograms of himself to simultaneous rallies in multiple cities and exploiting his 1.4 million-strong Twitter following.
Melenchon vowed to return "power to the people", promising a revamped European Union and a €100 billion ($107 billion) stimulus package.
An anti-imperialist admirer of Venezuela's late socialist leader Hugo Chavez, he has been loath to criticise its current government, accused by international powers of dismantling democracy.
After 30 years as a canny political operative, some have suggested his current prominent positioning is the result of astute manoeuvring as he benefits from disarray in the Socialist party.
He has also spotted a new opportunity in the infighting rocking Le Pen's National Front as it, too, struggles to decide on a direction after this year's election defeats.
He has urged those disillusioned with mainstream politics and globalisation to switch over from Le Pen's far-right to his hard-left, saying those who are "angry but not fascists" are welcome.
In the meantime, whether it is deploring cuts to subsidised jobs or demanding a parliamentary inquiry into poor preparations for Hurricane Irma, he seems determined to doggedly challenge Macron's agenda.
"This is where the opposing power is," wrote Alain Duhamel in the newspaper Liberation this week. "This is where the counter-presidency is."
By Katy Lee