James Craigen fired Partick to a 1-0 victory in the title showdown with second-placed Morton.
Craigen's strike, five minutes before half-time, moved Thistle five points clear in the race for the Division One crown.
The midfielder latched onto a cross from Christopher Erskine and tucked the ball home from close range for his fourth, and most vital, goal of the season.
Morton went on the attack after the break and Martin Hardie fired narrowly over the bar before Craig Reid curled his shot just wide.
But Thistle held out in front of a bumper Firhill crowd of almost 9,000 to take another big step towards the title.