An actuator with a position detection mechanism includes a main body, a transmission mechanism, a quick-release mechanism and a position detection mechanism. The transmission mechanism includes a worm gear with protruding keys and a lead screw shaft. The quick-release mechanism is mounted onto the lead screw shaft and includes a clutch gear with key slots for disengaging or engaging with the protruding keys in order to move axially relative to the worm gear. The position detection mechanism includes a variable resistor, an actuating gear mounted on the lead screw shaft to rotate altogether therewith and a driving gear connected to the variable resistor and engaged with the actuating gear. When the worm gear disengages from the clutch gear, a particular resistor value is formed at the variable resistor; therefore, disengaged or engaged positions of the worm gear and the clutch gear can be precisely controlled.