The 21-year-old has joined the club for an undisclosed fee.
Servais made 34 appearances for Mechelen last season. He scored twice as his side finished 5th in the Belgian Pro League.
Steve Gallen, Millwall's director football said: "He's young, but plays with real maturity for his age. We see him as a central midfielder - athletic, strong and comfortable playing progressively.
"Mathis has been on our radar for a year now, since his move to Mechelen in the Belgian top flight, and we've watched closely as he's progressed there and with the Belgium Under 21s.
"As with any player arriving from abroad, we'll need to be patient as he acclimatises to the team, country and culture, but the potential is exciting and we're really pleased to have him on board."
The Belgian becomes the Lions' third summer signing, after bringing in Taïryk Arconte and Jenson Metcalfe so far in the transfer window.