The Bhoys had taken the lead after just 20 minutes when the unfortunate Darren Dods put through his own net, but Dods put things right by grabbing an equalizer at the right end just six minutes later.
The Saints had looked like holding on for a richly deserved point until Larsson's late intervention with a splendid free kick from the edge of the box to give Celtic another late win after their last gasp victory over Kilmarnock last weekend.
The win lifts Celtic ten points clear of Rangers for 24 hours at least with The Gers entertaining Aberdeen on Sunday, while St Johnstone are still looking for their first win of the season.
Livingston got back to winning ways after their defeat to Rangers last week with an impressive 3-1 win over struggling Hearts at Tynecastle.
David Fernandez put Livvy in firm control with two goals in five minutes midway through the first half.
The Spanish striker opened the scoring with a searing drive in the 21st minute and he doubled the lead after 26 minutes. Barry Wilson made sure of victory with the third goal in the 72nd minute.
Canadian international Kevin McKenna pulled one back for the home side just four minutes later but it proved to be nothing more than a mere consolation for Craig Levein's men.
Dundee and Dundee United shared the spoils in the Tayside derby after a fiercely contested 1-1 draw at Dens Park. United had taken the lead after just 11 minutes through former Rangers midfielder Charlie Miller.
Dundee drew level four minutes before the break with a penalty from Argentine star Betto Carranza.
Chinese defender Fan Zhiyi endured a miserable derby for Dundee when he was sent-off for his second bookable offence midway through the second half, but the home side held on for a point.
Motherwell gave new boss Eric Black his first win in charge and moved out of the bottom three with a hard-fought 1-0 win over Dunfermline Athletic at Fir Park.
Greg Strong netted the only goal of the game after just 16 minutes to give the home side all three points.
Kilmarnock and Hibernian fought out a goalless draw in a disappointing game at Rugby Park.
Click on fixture for full match summary
Dundee 1-1 Dundee United
Hearts 1-3 Livingston
Kilmarnock 0-0 Hibernian
Motherwell 1-0 Dunfermline
St Johnstone 1-2 Celtic
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