The Mayor was responding to a question tabled by Liberal Democrat London Assembly member Caroline Pidgeon who asked for an update as to when the line will reopen and the continued grounds for its closure.
Mr Khan replied: "The Waterloo & City line and Central line operate a shared pool of train operators [drivers]. The Central line is one of the busiest on the network, and throughout the pandemic, has seen a smaller decline in customer numbers than other lines, which has enabled TfL to support the journeys of many people who are less able to work from home.
"As a result, TfL prioritised Waterloo & City line train operators to the Central line to ensure it can run a frequent and reliable service, enabling social distancing where demand remains comparatively high.
"TfL is continuing to closely monitor customer demand to inform its decision about when to re-open the Waterloo & City line. This includes monitoring daily arrivals at Waterloo mainline station, which have remained low, and demand on alternative routes between Waterloo and Bank."
Deputy Alastair Moss, chairman of the City of London Corporation's planning and transportation committee, this week revealed that the Square Mile's local authority has written to TfL commissioner Andy Byford to raise concerns about the continued closure of the tube link between Waterloo and Bank.
He said: "We need to be very clear with TfL that we expect it to be reopened.
"And we want to - in the nicest possible way - put them on the spot as to what the trigger point will be for them to reopen that very important line for us."