Johnnie Jackson's opener set the tone for a convincing performance, and the Championship club's other goals came from Danny Hollands, Yann Kermorgant and Danny Green.
St Mirren were in action for the first time this summer and almost went ahead through Kenny McLean early on.
But captain Jackson ran on to Kermorgant's pass and beat goalkeeper David Cornell to give Charlton a half-time lead.
And Charlton dominated after the break despite wholesale changes, Hollands combining with Lawrie Wilson before volleying his side's second.
Kermorgant's superb bicycle kick made it three and Green sealed victory with a powerful left-footed drive.