Virgil van Dijk's wonderful fifth-minute goal proved enough to secure a sixth successive league win for Neil Lennon's men, putting them 11 points clear.
The Dutch centre-back waltzed past two challenges before firing into the bottom corner, and the strike proved decisive as St Johnstone goalkeeper Alan Mannus held firm thereafter.
Fraser Forster, in the opposite goal, made one fine save from a vicious Gwion Edwards strike en route to a fourth successive league clean sheet.
Dundee United, Aberdeen and Inverness Caledonian Thistle, all 10 points behind Celtic at the start of the day, gave up ground on the leaders
United, who had won six successive matches, saw their fine form brought to a halt with a 4-1 loss at St Mirren.
Steven Thompson, the 35-year-old former Scotland striker, netted a brace against the club at which he started his career, while Keith Watson was sent off in the second half for the visitors.
Aberdeen lost 1-0 at home to Motherwell, who climbed three places to second as a result. Henri Anier unselfishly set up the only goal for Lionel Ainsworth.
Inverness Caledonian Thistle had goalkeeper Dean Brill to thank for a 0-0 draw at Partick Thistle that leaves them third.
The 28-year-old Englishman made a string of fine saves, including one from a terrific Ross Forbes strike.
Rock-bottom Hearts suffered a 4-0 home defeat at the hands of Kilmarnock.
Kris Boyd registered his 10th and 11th goals of the season – only Billy McKay of Inverness has scored more – but the star of the show was Chris Johnston.
The 19-year-old winger created both of Boyd's goals and scored a fantastic strike himself – his fourth in five games.
The result lifted Kilmarnock seven points clear of second-bottom Ross County, who lost 2-0 at home to Hibernian.
Defenders Michael Nelson and Jordon Forster ensured Hibs scored more than one goal in a game for the first time since September.
Ross County have now gone nine matches without a win and have lost their last five.
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