The 21-year-old clocked one hour and 19 minutes to dominate the 67km third stage which ran from Chiang Mai downtown via Sankam Phaeng district to Huay Kaew Arboretum.
Campbell whose previous win occured in Colombia last July settled at third in the Monday’s stage.
Coming at second was Marlen Reusser of Switzerland with Thi Thi Nguyen of Vietnam finishing third.
Thai hope Jutatip Maneephan, the first stage winner, took the fifth position. She, however, still remained on top in the individual general classification following a time of 3:48:56 hours, three seconds ahead of Campbell.
The third and final stage (83.80km) will start on Wednesday’s morning in Chiang Mai.