The Blues were under a transfer ban this time last year, but have emphatically made up for lost time this summer as they have undergone a major spending spree.
However, despite the significant outlay, manager Frank Lampard has still been in the market for a goalkeeper to compete with and probably claim the number one spot between the sticks from Kepa Arrizabalaga.
The Spaniard is the most expensive goalkeeper in history after Chelsea signed him for £71.6m from Athletic Bilbao.
However, he has failed to live up to the price tag, with his career at Stamford Bridge littered with high-profile mistakes.
Even though the transfer of Mendy from Rennes was in progress anyway, the final nail in the coffin for Kepa likely came on Sunday as he made a dreadful error in Chelsea's match with champions, Liverpool.
Taking too long to clear the ball, he attempted to clear possession out of the box with a weak pass, only for Sadio Mane to intercept and pass a finish home into the empty net.
As Rennes qualified for the Champions League last season, Mendy played 24 times in Ligue 1, keeping nine clean sheets and conceding just 19 goals.