As tension mounts between the rich northeastern region and the Spanish government, Mas told AFP in an interview how he wants his drive for Catalan independence to play out after the vote.
If pro-separatist candidates win, Catalonia will declare independence in 18 months or two years, he said, warning that Catalonia will not pay its share of Spain's debt if Madrid does not agree.
He said he did not want to make threats, but felt compelled to defend himself against a "campaign of intimidation" by opponents trying to "influence the vote".
The Spanish government, fiercely opposed to losing Catalonia, along with the big businesses and banks that would entail, have warned of economic and financial disaster if Catalonia secedes.
Roadmap to secession
Mas wants Catalonia to follow the example of Scotland by holding a referendum on independence, although in its vote a year ago Scotland voted not to break away from Britain.
The Spanish government has blocked that plan, so Mas has framed Sunday's election for the regional parliament as an indirect plebiscite on independence.
"Clearly, if we get a majority of the votes on September 27th, then that's the referendum done," he said in the interview with AFP in Barcelona on Tuesday
But short of winning an outright majority, he said he could still hold a referendum if the new government that emerges from December's Spanish general election changes Madrid's stance.
"If we won a majority of seats but not a majority of votes and the Spanish government offered us a binding referendum on independence - though I am very sceptical about whether it would do so - then we would listen," Mas said.
He said he hoped for an amicable separation from Spain, "a bit like children who grow up and move away from home but do not cut the ties of friendship and love with their parents."
Spanish Prime Minister Mariano Rajoy has said Mas would be depriving citizens of the right to be both Catalan and Spanish.
"I am convinced that most Catalans would retain both nationalities," Mas said. And he himself? "Why not?"
The latest opinion polls show pro-independence parties could win close to half of the vote and an absolute majority of seats in the Catalan parliament.
But Mas stressed that any declaration of independence will not come overnight even if his pro-independence list wins.
He said Catalonia would declare independence within 18 to 24 months after setting up the apparatus of a new state and adopting a constitution.
Catalans would be asked to vote on the constitution in a referendum which would also give them another chance to vote for or against independence.
Meanwhile, Mas said he intends to negotiate a friendly divorce from Spain and an agreement with the European Union for Catalonia to stay in the bloc.
"The Catalan state must be recognised as an EU member but that must be negotiated before independence," he said.
If it reaches an agreement with Madrid and Brussels, he said Catalonia would take on its share of Spain's debt.
"If there is no agreement, we will have no obligation to pay Spanish debt," he said, reiterating a warning he has made over recent days.
Catalonia accounts for 16 percent of Spain's population and a fifth of its economic output. Spain's debt is nearly equal to its own total gross domestic product, around a trillion euros.
"If it does not make an agreement with Catalonia, how will Spain be able to pay back its debts, which will rise to 120 percent of its output, while it loses the most productive part of its economy?" Mas said.
"If things get as complicated as that -- and there is no reason why they should -- it is the whole of Spain that will have a problem."
By Patrick Rahir, Daniel Bosque