23-year-old Kongolo joins the Monegasque side on a five-year deal for a fee in the region of £12m.
Manchester City have been making a big effort to bring Mendy to the club and it is understood that the player is keen on a move, with some sources even reporting that City have already come to an agreement with the player.
Pep Guardiola's side will be hoping that the signing of Dutch international Kongolo, who plays in the same position as Mendy will free up the French international to join their ranks in the near future.
Mendy was a key member of the Monaco side that won Ligue 1 and reached the semi-finals of the Champions League, whilst Kongolo was also a vital player for Feyenoord as they won an historic Eredivisie title.
The 'Daily Mail' reports that City could look to Southampton's England international left-back Ryan Bertrand if their efforts to sign sign Mendy do not make progress soon.