In an end-to-end affair in which both sides threw the ball around, Tonga's power up front eventually took them over the line.
While Tonga dominated the opening stages to pile on three tries in 22 minutes, Lebanon never gave up.
They remained within one try for much of the match - including an extended scoreless period in the second half - and made things interesting late on with a try-of-the-tournament contender from Abbas Miski.
Lebanon had three tries disallowed as they pushed to clinch a winner.
