And the 22-year-old, signed from Isthmian League Premier Division side Bognor Regis Town, bagged a brace on his debut for the Stags.
Mongoy joined Bognor in the summer of 2020 after returning from a spell in Sweden with IK Gauthiod.
He moved to Sweden from Bognor`s league rivals Lewes, who he had joined in June 2019.
He spent the second half of the 2018/19 season with Isthmian League South East side Hastings United, where he netted a total of 17 goals and 10 assists.
The versatile forward, who can play as a striker, on the left wing or behind the striker in the number 10 role, came through the youth ranks at Dagenham & Redbridge, before moving to Billericay Town.
He signed for Hastings in December 2018.
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