McGowan, 25, was expected by many to leave St Mirren Park when his contract expired this summer, with several English clubs reportedly interested in signing the former Celtic midfielder, but he allayed the fears of the Buddies fans when he signed a new one-year contract.
Saints supporters were handed another boost when Harkins signed a two-year deal with an option of a further season.
The 28-year-old midfielder departed Dundee after the Dens Park side were relegated from the Clydesdale Bank Premier League, after moving back to the Tayside club for a second spell in January.
St Mirren boss Lennon told the club's official website: "I am delighted that we have the signatures today of Paul McGowan and Gary Harkins.
"We have a clear vision of what we want to achieve and how we want to achieve it this season and these two players are key to those plans as they are both outstanding, quality footballers who are thrilling to watch and can influence games.
"We have other players we want to bring in to complement them and we want to see some of their qualities rubbing off on the young players we are developing here.
"We are pulling together an exciting squad for the coming season and I am already itching to get back on the training ground with them."
The Paisley club also confirmed that keeper Grant Adam has cancelled his contract by mutual consent.
- Sports & Recreation
- Clydesdale Bank Premier League
- Gary Harkins