The 19-year-old Plummer, a member of City’s elite development squad, will gain more first-team experience with the Scottish Premiership club after his temporary move to Oldham Athletic.
Plummer was loaned out to the Latics in November, making three appearances for the League One side, and City are hoping he can play a key role for the Buddies until the end of the campaign.
Patrick Vieira, head of the elite development squad, told the club’s official website: “This is an exciting development opportunity for Ellis. The experience of playing in the Scottish Premiership will be of huge benefit to him.
“I’m looking forward to seeing the progress he makes up in Scotland and we wish Ellis and St Mirren well for the coming season.”
Plummer made 17 appearances for Vieira’s side last season and was nominated for the Academy Player of the Year award.
Meanwhile, midfielder McLean - who has made more than 100 appearances since making his debut four years ago - has agreed a two-year contract after his most recent deal ran out this summer.
He told the club's website: "Having looked at various options I decided it would be better to stay at St Mirren and continue my development here."