Mahlon, 20, initially joined The Gills the summer of 2012 on a two-year scholarship, having previously played in the youth systems at Arsenal and Wycombe Wanderers.
The right-back was given permission to feature a couple of times for Millwall's U21s towards the end of the 2014/15 campaign and when injuries to both Carlos Edwards and Shaun Cummings handed him a first-team chance at Walsall in February 2016, he grabbed it in spectacular style. Not only did The Lions romp to a 3-0 win at the League One high-flyers, Mahon marked his debut with a stunning goal.
His assured displays in the back four saw the young prospect hold down the right-back spot until a harsh red card at Gillingham on the final day of the regular season kept him sidelined for the Play-Off games and an appearance at Wembley.
There is no doubt that the future looks bright for Mahlon, who will be determined to continue his excellent progress throughout the 2016/17 campaign.