The 21-year-old spent the second half of last season with the Paisley club alongside fellow Magpies recruit Paul Dummett and helped them secure their first major trophy in 26 years after scoring against Hearts in a 3-2 League Cup Final success.
He was sent back north by United boss Alan Pardew in the summer and has now been joined by another Newcastle youngster, striker Adam Campbell, as part of an affiliation pact between the clubs.
St Mirren chief executive Brian Caldwell confirmed the deal, saying: "Conor Newton has this afternoon extended his loan spell with St Mirren until the end of the season."