The highly rated 19-year old defender has agreed to join Tommy Craig's side on-loan until the end of the season.
This will be Plummer's second loan spell after he joined Oldham Athletic on a youth loan for five weeks in November 2013.
"This is an exciting development opportunity for Ellis, the experience of playing in the Scottish Premier League will be of huge benefit to him," said Head of Elite Development Squad, Patrick Vieira, to City's website.
"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."
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