The visitors, seeking a return to top flight Scottish football for the first time in a decade, sealed their promotion thanks to second-half goals from Andy Dowie and Chris Erskine.
Conor McGrandles gave the 2,000 travelling supporters an early fright when he forced Thistle keeper Scott Fox into action.
But Alan Archibald's men soon took command and Conrad Balatoni looked sure to open the scoring but headed wide from close range.
Ross Forbes then struck the goalframe with a venomous left-footed free-kick from 20 yards two minutes later as the Jags continued to dictate terms.
The breakthrough duly arrived five minutes into the second half when Dowie headed home Forbes' corner and Erskine wrapped up the contest with an assured 78th-minute finish.
Archibald, who took interim charge in January following the departure of Jackie McNamara, said: "One thing we have is a young team with a lot of hunger and desire and they want to prove themselves. That's what drove them on all season.
"The fans have been brilliant all year and they have been the 12th man and got us over the line on a number of occasions."