The pair, who both came out of retirement to participate, provided an Egyptian-themed routine in the preliminary round, scoring 82.333 points, ranking eighth but qualifying for Thursday's finals.
Leclair, who retired in November, won a gold in the team event at the 2011 Pan Am Games and also has two bronze medals at the FINA World Championships to her credit.
The 25-year-old, from Gatineau, Que., also competed at the London Olympics in the team event.
The pair were chosen at a selection trial in June to compete in the world championship, as the mixed duet makes its FINA debut.
Leclair and the 35-year-old Prevost are in competition with some synchro swimmers with pedigree including Olympic bronze medallist Virginie Dedieu and two-time silver medallist Gemma Mengual.
The U.S. team, consisting of Bill May Kristina Lam-Underwood are in first after Tuesday, while Russia's Aleksandr Maltsev and Darina Valitova are in second. France's Dedieu and Benoit Yves Beaufils are in third.