The visitors enjoyed a deserved half-time lead having created the better of the few chances in the opening period.
Garner and Paul Gallagher had both produced saves from Jamie Jones but the Orient keeper was left helpless when garner rifled North End in front.
Keith Keane delivered a cross for the right and Garner volleyed a ferocious right-footed effort into the top-left corner to collect his fifth goal of the season.
Preston keeper Declan Rudd was rarely called in the opening 45 minutes but after the interval, the home side increased their tempo but although they totally dominated the final 25 minutes, they found the visitors defence in resilient mood.
A last-ditch tackle by Bailey Wright denied David Mooney a realistic chance but the best opportunity fell to substitute Robbie Simpson who raced clear of the North End defence deep into added time but saw his shot blocked by Rudd.
The defeat cost Orient their place at the top of League One with Wolves taking over top-spot and it was the first time Russell Slade’s side have failed to score this season.