It is reported that Chelsea have paid £22m for the Senegal international.
"The medical I think is done and complete," Lampard told reporters. "I want to be a bit careful with that.
"I don't want to give absolute confirmation, but possibly over the next 24 hours. It seems like one which is done."
Mendy has been recruited to replace the struggling Kepa Arrizabalaga.
The 25-year-old was given a chance to prove himself again this season, but he has already made two mistakes in games against Brighton and Liverpool.