The British duo dominated from the start as they finished 2.62 seconds ahead of France with Germany in third.
Two other Coleraine men Alan Campbell and Peter Chambers failed to reach the A finals in their events.
Single sculler Campbell and lightweight four team member Chambers will compete in B finals on Sunday.
The lightweight pairs is a non-Olympic class but the performances of Cassells and Scrimgeour could put them in contention for promotion to one of Britain's Rio berths for next year.
Earlier this season, Cassells and Peter Chambers won the European title in the lightweight pairs class.
Chambers and his team-mates could only manage fourth in their lightweight four semi-final with Campbell fifth in his single sculls semi-final in Aiguebelette.
The Coleraine men will attempt to secure Olympic Games spots in Sunday's B deciders.
The top five finishers in the lightweight four B final will clinch places in Rio for their countries while Campbell, 32, will need a top-three finish in his B decider to book a British berth in Brazil.
2002 Olympic bronze medallist Campbell, who missed out on selection for last year's World Championships, was always going to be up against it in Friday's semi-final.
London Olympics winner Mahe Drysdale powered to victory in 6:45.08 which left him ahead of this year's European champion Damir Martin of Croatia with 2008 Olympic gold medallist Norwegian Olaf Tufte edging out Belarus's Stanislau Shcharbachenia to take the final qualifying spot.
Campbell was a further 3.21 seconds back in fifth spot.
Chambers and his British team-mates Jonathan Clegg, Mark Aldred and Chris Bartley were never really a factor in their semi-final as they finished 7.33 seconds behind Switzerland with France and Italy also progressing to the A final.
The British quartet's performance continued their recent struggles after they also failed to make the A final at the recent Lucerne World Cup regatta.
Richard Chambers and Will Fletcher will race for Britain in the lightweight double sculls final on Saturday.
Ennisillen woman Leonara Kennedy and her Irish team-mate finished a distant fifth in the final of the non-Olympic women's four final on Friday.
Coleraine rower Joel Cassells and his British team-mate Sam Scrimgeour have won the lightweight pairs gold medal at the World Championships in France.