[1] Den Hartog played in the youth academy of RKC Waalwijk after having been scouted at his local club HSSC '61.
[2] After being promoted to the first team of RKC, he was unable to play for a long time due to a fracture of his calf and shin.
Den Hartog finally made his debut for the Waalwijk-based club on 19 December 2014, when they lost 5–3 to Emmen.
[4] That season he would make six appearances for the main squad and in the summer of 2015 he was assigned jersey number 1.
[5] After this loan period, Kozakken Boys decided to sign the goalkeeper on a permanent contract.