When the window shut at 11pm on Friday it was not clear if the 23-year-old winger's loan move to Amiens had gone through.
The Ligue 1 club announced he had joined them for the rest of the season. The paperwork was completed before the deadline, but the deal was yet to be signed off by the relevant authorities.
Mbenza's departure completes a total overhaul of the squad carried out by manager Danny Cowley in his first transfer window.
Seven players were signed, although Kieran Phillips joined from Everton more with a view to initially going into the under-23 squad, and eight left. Seven of the departures were on loan, and Aaron Mooy's temporary move to Brighton and Hove Albion was made permanent at a cost of £5m.
Importantly, the Terriers also saw off interest in strikers Karlan Grant and Steve Mounie.
Cowley signed free agents Danny Simpson and Fraizer Campbell before the window opened.
Mbenza's only start league start came under Jan Siewart. He made just two substitute appearances under Cowley, the last on October 1.
Amiens are in danger of relegation after 19 points from their first 22 games. They are in the division's relegation place.
The Belgian Under-21 international only joined permanently in the summer, having been on loan the previous season, when the Terriers were in the Premier League. His only goal came in May's 1-1 draw against Manchester United.
Meanwhile, former player Danny Schofield has returned to the club as an under-19 coach. he replaces former Huddersfield and Middlesbrough midfielder Dean Whitehead, who left for Shrewsbury Town late last year.
Schofield, now 39, scored 46 goals in 285 appearances for Town as a winger, and was part of the side that won promotion from the third tier in 2004. He also played for Rotherham United, Halifax Town.and Bradford Park Avenue.
Schofield, who previously coached Middlesbrough's under-23s has a sports science degree and a Uefa A licence.