McMillan, who was included in PFA Scotland's First Division team of last season, is no stranger to Firhill having spent the closing stages of last season at the club following his departure from cash-strapped Dunfermline Athletic.
The 24-year-old played three times in an injury-hit period as Thistle won promotion back to the Scottish Premier League but said: "I'm really looking forward to the challenge ahead.
"I enjoyed my time here towards the end of last season and you get a feel for the place and you feel part of the club.
"We will need to go into games with the belief that we can win them. Doing well at home will be the most important thing and we had an excellent record at Firhill last season and that's something we will need to try and continue.
"I know people are writing us off already but we've a young squad of players who will be determined to go out and prove themselves.
"It's obviously a blow that we've lost players like Chris Erskine and Paul Paton. It would be a blow to lose any player who has played the number of games that they have. That though is just part and parcel of football.
"The manager has already brought in a few players; players with SPL experience and I'm sure that he'll be looking towards bringing in other players as well."
The club later announced that Hall of Fame inductee Neil Duffy, who served Thistle with great distinction during the 1960s making 190 appearances and scoring 67 goals, had passed away.