Lokeren was trailing to David Pollet's 63rd-minute strike, which would have kept alive Mons's hopes of snatching sixth after its win over Cercle Brugge, until Persoons netted a minute from the end to give his side a four-point cushion with one match remaining before the 30-game split.
Mons was a 3-1 victor at Jan Breydel Stadion, Mustapha Jarju and Aloys Nong adding to Gregory Mertens' own goal.
Substitute Joaquin Boghossian pulled a goal back for the bottom side, which will go into the five-game relegation play-off facing a three-point deficit to Beerschot.
The latter will finish the regular season 15th after it lost 3-2 to Waasland-Beveren and 14th-placed Lierse beat Kortrijk 1-0.
Benito Raman and Hernan Losada's goals for Beerschot were in vain as Barak Badash's penalty and goals from Jordan Remacle and Stijn de Smet gave its host victory.
Rachid Bourabia scored Lierse's goal before Kortrijk had Stefan Mitrovic sent off late on.
Anderlecht will finish the 30-game regular season at the top of the table after a 1-0 win over KV Mechelen, the goal scored by Tom de Sutter's goal, coupled with second-placed Zulte-Waregem losing 1-2 at home to Club Brugge.
Carlos Bacca and Eidur Gudjohnsen earned Der Club a 2-1 win, Olafur Ingi Skulason with the late reply for Zulte.
Third-placed Genk beat Gent 3-2. Thomas Buffel's opener was overhauled by Rodger Kola and Ilombe M'Boyo but Bennard Kumordzi equalised and Julien Gorius grabbed the winner.
Standard Liege shrugged off the dismissal of Jelle van Damme just after half-time to beat Leuven 4-0. Imoh Ezekiel and Michy Batshuayi struck in the first half and Laurent Ciman and Frederic Bulot added further goals after Van Damme's dismissal.