The former Walsall forward levelled on the stroke of half-time after Calum Elliot headed the hosts in front just seconds earlier.
New Hearts strikers Kevin Kyle and Stephen Elliot were given a standing ovation when they made their first appearances as 65th-minute substitutes but the pair failed to influence the outcome of this Scottish Premier League opener.
The visitors held on at the end despite being reduced to 10 men in the 74th minute when Steven Anderson was dismissed for a rash tackle on Suso Santana.
Hearts were fast out of the blocks and created the first chance after just 30 seconds. Ian Black's inch-perfect cross-field pass found David Templeton and he cut inside on his right-foot from the left flank before pulling a low shot just wide from 18 yards.
The pint-sized winger had the ball in the back of the net moments later but he was adjudged to have been in an offside position as he accepted Suso's pass.
St Johnstone had their first sight of goal when Michael Duberry's header from Liam Craig's corner caused problems, but Hearts breathed a sigh of relief as the ball cannoned off Danny Grainger for a goal kick.
Another teasing delivery from Craig picked out Chris Millar at the back post but his tame volley on the stretch was well saved by Marian Kello.
That seemed to spur Hearts into action as Elliot struck a low effort that debutant goalkeeper Peter Enckelman got down well to.
The away side were dealt a blow in the 20th minute when Grainger had to be carried off with his right leg in a brace after colliding with the side-netting as he flicked Craig Thomson's free-kick out of Elliot's reach, and Cleveland Taylor came on for his debut.
After a Hearts' counter-attack broke down when Craig Thomson failed to find Elliot, the midfielder then fed Ruben Palazuelos who swivelled and hit a left-footed strike just over the crossbar.
In a frantic ending to the first half Hearts took the lead in the 44th minute through Elliot but Saints drew level within 60 seconds.
Firstly the hosts broke the deadlock when Elliot met Lee Wallace's cross at the near post to head past Enckelman.
However, the visitors were level courtesy of a debut goal from Parkin after he met Taylor's far-post cross.
Craig Thomson's driving cross from the right that was destined for Elliot was headed over his own crossbar by Duberry as the hosts started the second period more brightly.
Kyle and Elliott made their entrance to a roaring welcome with 25 minutes to go and the change in atmosphere helped swing the momentum after the visitors had enjoyed a concerted spell of territory.
St Johnstone were down to 10 when Anderson recklessly lunged into the back of Suso, who had earlier been booked for diving, on the near-side touchline.
Hearts' numerical advantage saw Saints play with Parkin as their lone marksman, with Chris Millar dropping back to compensate for Anderson's absence.
Hearts were looking to hit high balls into Kyle but commanding defender Duberry superbly marshalled his defence and the former Chelsea player was on hand to head the ball off the line in the 85th minute as Kyle's looping header seemed goalbound.