Passengers will need a prepaid or concessionary ticket, Oyster card or a contactless payment card to travel on London's buses from today.
TfL said only 0.7 per cent of all bus journeys were paid for with cash and that tourists were unlikely to be affected.
TfL said its research showed the move was unlikely to affect tourists "as the vast majority use a prepaid ticket, such as a Visitor Oyster, to get around the capital".
"Removing cash from our bus network not only offers customers a quicker and more efficient bus service but it enables us to make savings of 24 million pounds a year which will be re-invested to further improve London's transport network," Mike Weston, TfL's director of buses, said.
Also Read
The removal of cash fares will deliver 130 million pounds savings to 2022/23 and this will be reinvested in improving transport in British capital, TfL said.
Johnson noted that more than 2,100 Oyster cards were lost, stolen or stopped working on the average day last year.
However, TfL defended its action, saying the decision to move to cash free bus travel follows a considerable drop in the number of people paying their bus fare in cash.
A decade ago around 25 per cent of journeys were paid for with a cash fare with that figure now falling to less than one per cent, TfL said.