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.

Users were angry that clicking the cross to dismiss the box meant that they had agreed to the upgrade.
Based on "customer feedback", Microsoft said that it had added another notification that provided customers with "an additional opportunity for cancelling the upgrade".
The pop-up design had been described as a "nasty trick".
Microsoft told the BBC it had modified the pop-up two weeks ago as a result of criticism: "We've added another notification that confirms the time of the scheduled upgrade and provides the customer an additional opportunity for cancelling or rescheduling the upgrade.
"If the customer wishes to continue with their upgrade at the designated time, they can click 'OK' or close the notifications with no further action needed."
Senior editor at PC World magazine Brad Chacos, who describes himself as a fan of the Windows 10 operating system, had previously described the use of the cross to mean people had agreed to the upgrade as "a nasty trick".
He said: "I don't think that adding more pestering pop-ups improves the situation. At the very least they should add a large, obvious 'No, I don't want this' button."
Mr Chacos has been a vocal critic of what he described as the "heavy-handed tactics that Microsoft's been using to force people into the upgrade".
Previously, users had to press the cross to cancel the suggested upgrade, he pointed out, so the latest move is counter-intuitive - "akin to swapping out the brake and the accelerator in your car".
Users have been sharing with the BBC their own experience of unwanted upgrades, suggesting that the process is difficult to cancel once it has begun.
Havard Hughes' experience was typical of many.
"Windows 10 update ran on my PC despite all my efforts to stop it, including dismissing the update several times and frantically trying to abort the installation as it started when I was halfway through writing an important e-mail.
"My attempt to roll it back to Windows 7 resulted in the blue screen of death and a dead PC.  I now have to reinstall my home computer from scratch because of this so-called 'free' upgrade.
"As someone who paid for my software and was perfectly happy with my fully functional Windows 7 machine, this has been an absolute disaster."
Coleraine rower Joel Cassells and his British team-mate Sam Scrimgeour have won the lightweight pairs gold medal at the World Championships in France.
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Microsoft has u-turned over changes it made to a pop-up encouraging users to upgrade to Windows 10.