Former Celtic youngster McGowan had been attracting interest from a number of clubs in the English leagues, but put pen to paper on a contract extension this morning, keeping him at the club until the summer of 2014.
The influential midfielder has made 115 appearances for the Saints so far, scoring 21 goals in the process.
St Mirren also beat off competition to secure the services of former Blackburn Rovers midfielder Harkins. The 28-year-old has signed on until summer 2015, with the option of a further year.
He spent the latter part of last season with Dundee after being released by Kilmarnock.
• Grant Adam has also cancelled his contract with the Paisley side.