President of Catalonia from 2010 to 2016 when he was forced out, Mas stood trial in February along with two former members of his government, accused of serious civil disobedience and misconduct for organising a symbolic, non-binding referendum in 2014 despite a court ban.
A hero to some Catalan nationalists and an unscrupulous fraud to their opponents, Mas was once described by centre-right Spanish newspaper El Mundo as "the technocrat who turned into the Catalan Odysseus."
But for Mas, he is but a "public servant."
"Serving a country doesn't require martyrs or heroes, it requires resolute people who are firm in the defence of their convictions," he told reporters before a trial that has stoked pro-independence fervour in Catalonia.
Some 40,000 people turned up to support him and his former associates at the start of the trial, many of them waving separatist, red and yellow flags.
Mas outside court on Monday with thousands of supporters. Photo: AFP
"He wanted to stand in front of the crowds again and the high court made this easy," the El Confidencial online daily said in an opinion piece.
"Today, when he leaves the courtroom, he will have the support of those who had started to forget him."
Rise to separatism
Observers wonder at how a moderate conservative, upper middle-class economist came to trigger the wrath of Spain - and its courts - by trying to break its richest region away from it.
With his spectacles and neatly brushed quiff, the 61-year-old father of three started out as a businessman and then served in the regional government before being elected its president in 2010.
He has said that as a youth he was not linked to Catalan nationalism.
But after his separatist alliance and another pro-independence group won an absolute majority of seats in the Catalan parliament in a 2015 regional election he bellowed: "We have won."
According to Jordi Amat, a writer specialising in Catalan nationalism, the turning point for Mas was in 2006 when he negotiated political deals including a new statute giving Catalonia greater powers.
The agreements with Spanish leaders ultimately fell through and the feeling of betrayal hardened Mas's nationalism.
Carefully measuring his words in fluent Catalan, Spanish, French and English, Mas trod a cautious line at first as president, trying to negotiate more fiscal autonomy for the northeastern region.
But in September 2012, at the height of Spain's economic crisis, more than a million Catalans filled the streets of Barcelona demanding the right to self-determination.
For months he nevertheless avoided using the word "independence." It was not until 2013, after Madrid had rejected his fiscal demands, that he first publicly said he would support secession.
He initially wanted a referendum on independence like the one organised in Scotland, but shifted tack in the face of legal challenges from Madrid, settling for the 2014 symbolic vote instead, organised with the help of thousands of volunteers.
More than 80 percent of those who cast their ballot in the 2014 vote did so for independence - although just 2.3 million people out of a total of 6.3 million eligible voters took part.
But that still landed him in trouble and ultimately on trial.
After the vote, Mas called early elections for the regional parliament in September 2015, which he framed as an indirect independence ballot.
He ran in an alliance with leftwing nationalists and other pro-independence groups, which won.
His victory was short-lived, however, as the alliance's small, far-left CUP party refused to agree on a government with Mas as president, resenting spending cuts he implemented during Spain's recession and corruption scandals linked to his party.
So he stepped aside as leader in January 2016, naming Girona mayor Carles Puigdemont as his replacement, who has pledged to hold a referendum in 2017 - a binding one this time.
By Marianne Barriaux / AFP