The Pars had defender John Potter sent off after 61 minutes at McDiarmid Park on Saturday as he conceded a penalty for a trip on home debutant Francisco Sandaza.
But, with keeper Paul Gallacher saving Liam Craig's spot-kick, the Pars had Andy Kirk's first-half strike against the run of play - his third goal in as many games - to thank for their first three points back in the Premier League.
The win means last season's First Division champions remained undefeated for the campaign after draws against St Mirren and Inverness and ensured they were still unbeaten at McDiarmid Park in 16 years on league duty.
St Johnstone's failure to break down their opponents leaves them without a goal or a win since the season restarted.
Fit-again summer signing Sandaza was made to wait for his debut after starting on the bench but he almost inspired a second-half comeback.
But Perth manager Derek McInnes watched as his side's lack of a cutting edge again haunted them.
In a lively opening on a lush green surface, St Johnstone had the ball in the net in the ninth minute when Carl Finnigan deflected Craig's free-kick into the net.
But the former Falkirk man was booked for using a hand as referee George Salmond spotted the infringement.
The disallowed goal buoyed the home side, who continued well on top, and Gallacher had to react smartly to to keep out David McCracken's close-range header from Craig's inswinging corner.
Sheridan then screwed his shot on the turn wide from the apex of the six-yard box after Dave Mackay's throw-in had been touched on by Marcus Haber.
And the home side were then left to rue not earning the breakthrough when they were on top as they fell behind to their visitors on the half-hour mark.
A well-worked short corner was shuttled to Jason Thomson in acres of space on the edge of the box and his low shot was touched into the net by Northern Ireland international Kirk from six yards out for a goal well against the run of play.
Saints midfielder Craig, playing at left-back in the absence of Callum Davidson, was again the source of the home side's attacking play seconds after the restart but Murray Davidson's looping header dropped the wrong side of the crossbar.
With both sides struggling to find a route to goal, Dunfermline came close to a second goal in the 60th minute when a header from former St Johnstone midfielder Martin Hardie slipped just wide from Joe Cardle's corner.
But Sandaza then brought the match to life with a coruscating run which ended when he was brought down in the box by the trailing leg of Potter, who was red-carded for the foul.
However, with Sandaza asking manager Derek McInnes whether he should take the spot-kick, Craig stepped up and saw his effort blocked by Gallacher low to the keeper's left.
Sandaza again looked a threat in the 71st minute as he found space in the Dunfermline box but his header from a Jody Morris chip was held on his line by Gallacher.
The Pars rearguard was now hard-pressed as, with their extra man, Saints held the upper hand on possession.
However, they continued to hold firm and would have been relieved to see home substitute David Robertson flick his header from Craig's corner just too high from his near-post position.
With nine minutes left, Gallacher again proved an immovable last line of Dunfermline's defence as he held Davidson's net-bound header.
And Saints' chances of salvaging anything from the match slipped away as Craig drove a late free-kick too high.