Defensive midfielder Mendes has put pen to paper on a five-year contract with Lille and follows in the footsteps of Luiz Araujo, who left Sao Paulo for the Ligue 1 club in June.
Mendes won Brazil's Serie B with Goias in 2012 and, after two seasons in the top flight, moved to the Paulista giants and played a key role in their run to the 2016 Copa Libertadores semi-finals.
The 25-year-old expressed his delight at securing a European move and getting the chance to work under head coach Marcelo Bielsa.
"Since I was a child, playing in Europe has always been a dream for me," Mendes told Lille's official website.
"I am very happy to be at Lille and in a competitive league. I hope to achieve my goals and win titles with this shirt.
"I want to grow as an individual and as part of the team, especially with Marcelo Bielsa, who has a very high reputation. I hope to progress alongside him."
Lille also announced the arrival of the right-sided player Malcuit from Saint-Etienne on a five-year contract.
Malcuit had been linked with a move to Manchester United, who are said to have scouted him during the 2016-17 campaign.
Instead, he is hoping to establish himself as a player of Ligue 1 quality after spells in the third and second tiers with Monaco, Vannes, Frejus Saint-Raphael and Chamois Niortais between 2011 and 2015.
"I'm happy to be here wearing this jersey," he said. "I look forward to being on the pitch and evolving by working with Marcelo Bielsa.
"He is a great coach known throughout the world, who knows how to develop his players and push them to their limits."