The winger was brought down by former Bee John Mousinho in the box deep into injury time and got up to send goalkeeper Declan Rudd the wrong way from the spot.
It was reward for a dominant display from the Londoners who laid siege to the visitors' goal almost from start to finish.
Rudd kept North End in it with a string of first half saves to twice deny Clayton Donaldson while defender Harlee Dean also saw a header cleared off the line.
Match-winner Saunders curled an effort just wide of the far post before the break while Lee Holmes fired over and Jack King side-footed wide for Preston.
Brentford took control after the break but again it was Rudd who rescued his side, tipping Saunders' curling free kick to safety and then looking on as the same player fired wide after a strong Shaleum Logan run and cross.
Skipper Jonathan Douglas fired over from close range and Bradley Wright-Phillips blazed over as the clock ticked down.
But the moment of madness from Mousinho cost his side dear and Saunders kept his cool to take maximum points for the Londoners.
