Sadiq Khan, the current mayor, announced the vehicles had been sold to a firm which will dismantle them and export the parts, adding “we have finally managed to get rid of them”.
Mr Johnson bought the machines from Germany in 2014, before their use had been licensed on the UK mainland.
Theresa May banned them in riot situations when she was home secretary in 2015, and later used the purchase to ridicule Mr Johnson in the Tory leadership race after David Cameron resigned.
The water cannon cost £322,000 to purchase second-hand and then refit – including £32,004 for low emission zone compliance, £19,035 for re-painting, £3,109.20 for signage, and £970.50 for the fitting of radios and CD players.
They have now been sold for just £11,025 to Nottinghamshire-based Reclamations (Ollerton) Ltd, who will dismantle them and export the parts.
Mr Khan said: “For too long, London taxpayers have had to bear the brunt of Boris Johnson’s appalling botched water cannon deal. This has been another waste of taxpayers’ money by Boris Johnson.
“Londoners continue to live with his vanity. I am pleased we have managed to finally get rid of them and I made an election promise to Londoners that I would claw back as much of this cash as possible, and pump it into helping young people at risk of being affected by crime and giving them better life opportunities.”