The zone would require vehicles entering it to emit zero or low emissions or face a daily fine.
London’s air pollution is among the worst in Europe and campaigners have previously called for tougher action to improve public health.
Mr Johnson is widely expected to exit City Hall at the 2016 Greater London Authority elections, meaning responsibility for delivering the zone would fall to his successor.
Speaking at City Hall on Wednesday, the Mayor said: “Creating the world’s first big city ultra low emission zone has the potential to be a game changing moment in the quality of life of our great capital.
“My vision is a central zone where almost all the vehicles running during working hours are either zero or low emission. This would deliver incredible benefits in air quality and stimulate the delivery and mass use of low emission technology.”
Other measures outlined by the Mayor include introducing 1,600 hybrid buses by 2016 and establishing a £20m fund to help boroughs tackle bad air hotspots.
London Assembly Members first called on the Mayor to introduce an ultra low emissions zone in 2009.
Green Party AM Jenny Jones said the scheme would be “excellent news for Londoners’ health”, but said by leaving its implementation to his successor Johnson was “ducking responsibility for the problems we’re facing now.”
She added: “We have argued for years that the Mayor’s list of little measures is inadequate and that an ultra low emission zone was essential. It’s already too late for our breaking the European legal limits, but if we are to clean up London’s dirty air we need this zone now, with a zero emission target.”
Jenny Bates, Friends of the Earth London Campaigner said: “2020 is far too late for the Mayor’s plan for a Ultra Low Emission Zone in central London – EU legal limits must be met by 2015 for NO2, but London is set to still exceed levels required for this toxic gas until 2025.
“The Mayor must take bold and urgent action now including measures to cut traffic levels such as considering London-wide road user charging and drastically increasing funding for cycling – and must abandon proposed new road river crossings in east London which would just add to the traffic and pollution problem.”