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The 21-year-old centre-half has agreed a deal with the Paisley club until the summer of 2022.
Buddies boss Jim Goodwin told the club website: "Conor ticks all the boxes - with Sean McLoughlin going back to Hull it was a position that we had to bring in at least one."
St Mirren have also brought in midfielder Jamie McGrath from Dundalk and defender Akin Famewo has joined on loan from Norwich City.
