The visitors started off the match top of the league but did little to suggest they expected to stay there. The Buddies spent most of the first half passing back towards their 'keeper. It was only when they went two down that they showed some real intent and Simon Lappin was unfortunate to see a superb free-kick come back off the face the bar.
Celtic had already rattled the woodwork through Aiden McGeady's right-foot shot before they took the lead. It was a simple goal with McManus guiding home a header from Shunsuke Nakamura after 'keeper Chris Smith was unable to reach the cross. Smith did better when blocking Maciej Zurawski's shot after Nakamura again created the chance.
At the start of the second half Lappin's inswinging free-kick was headed wide by Garry Brady for the visitors before Nakamura responded with a free-kick which found the side netting.
Celtic went two up after McGeady's advanced on the Buddies goal before cleverly finding Petrov who drilled his shot home at the near-post.
St Mirren nearly pulled one back, but afer Lappin's free kick struck the bar, John Sutton was unable to force the ball home.
Kenny Miller should have broken his scoring duck and put the game well out of the reach of the visitors, when subsitute Craig Beattie's cross found him unmarked at the far-post but the striker steered his volley wide.