The 25-year-old defensive midfielder joins the French outfit for a fee thought to be worth up to €9m (£8m), putting pen to paper on a contract that will keep him at the club until 2022.
The switch sees him follow in the footsteps of Luiz Araujo, who also swapped Sao Paulo for Lille in June.
"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."