Terry, who arrived just over a year ago from Yeovil, cashed in on some excellent play by Adam Boyd in the 73rd minute to give Orient the lead.
Boyd earned his own reward six minutes later when he struck from the spot as Walsall crashed to their first home league defeat of the campaign following a disappointing display.
Walsall had scored five goals in their previous match against Southend but never looked capable of reproducing that kind of form against a well-drilled Orient side.
Walsall captain Anthony Gerrard wasted a golden chance to break the deadlock in the 69th minute when he headed a Chris Palmer corner just wide.
It was a crucial miss as Orient struck just four minutes later to signal the beginning of the end for Walsall's challenge.
Terry's strike owed everything to the persistence of the hard-working Boyd.
The former Luton Town striker muscled his way past Manny Smith before feeding the ball into the path of Terry, who made no mistake with a smart volley from 10 yards.
Walsall never recovered from that setback and their fate was quickly sealed as Orient - who had scored four at Southend in their midweek cup win - strolled to their second league victory of the season.
Winger John Melligan had caused chaos down the Walsall left flank all afternoon and one pacey run proved too much for full-back Paul Boertien, who sent the Irishman crashing in the area.
Boyd comfortably converted the penalty for his fourth goal of the season by sending goalkeeper Clayton Ince the wrong way with his spot-kick.