Mustapha Jarju's first half strike opened Mons 2-1 win over Waasland-Beveren in Sunday's final promotional round of Division Two to send Mons to the Belgian top-flight football next season.
The neutral ground game scheduled in Tubize Leburton, which marked Mons returned to the Belgian Division 1 after two years, has also seen equal contenders Waasland cancelled the scores, a few minutes after half time.
But Timmerman's 86th minute strike helped sealed Mons promotion hope to beat Wassland-Beveren who needed just a single point to qualify, to the contest. Both sides completed a record of 13 points with Mons edging them out on a healthier goal difference.
After finishing as 4th highest goal scorer in the Belgian division two and leading goal scorer for Mons with 18 goals to his name, the Gambian mid-fielder Jarju Sunday's opener has also helped him to complete a set of another 3 goals during the qualifiers.
AS Eupen and Lommel United who made the four finalist in this season's play-off also lost the battle for promotion to Mons.
