The 61-year-old, Catalan leader from 2010 until last year, was convicted of civil disobedience for organising the symbolic, non-binding poll in the wealthy northeastern region in defiance of a a ban by Spain's Constitutional Court.
Catalonia, a region with its own language and customs, has long demanded greater autonomy and separatists have for years tried to win approval from Madrid for an independence vote.
Spain's economic downturn has boosted the demands, with many Catalans resenting the taxes they pay to the central government in Madrid to subsidise poorer regions.
Anti-independence parties had asked for a legal opinion on the region's 2017 budget, which includes provisions for a referendum promised by Catalan President Carles Puigdemont.
In the November 2014 vote called by Mas, more than 80 percent of those who cast a ballot chose independence, but just 2.3 million out of 6.3 million eligible voters took part.
Calls for outright independence have increased in recent years. Polls show that Catalonia, which accounts for almost a fifth of Spain's economic output, is roughly split in half over breaking away.