The plans include a ban on all adverts for food and drink high in fat, salt or sugar across the entire Transport for London network, including buses.
Khan said: "I am determined to do all I can to tackle this issue with the powers I have and help Londoners make healthy food choices for themselves and their families".
Khan is to consult on proposals to ban advertisements for unhealthy food and drink that are high in fat, salt or sugar across the entire Transport for London (TfL) estate, including the London Underground, London Overground, and the capital's buses and bus shelters. The model is now used by the UK's Advertising Standards Authority and its communications regulator Ofcom.
"It can't be right that in a city as prosperous as London that where you live and the income you have can have a massive impact on whether you have access to healthy, nutritious food and your exposure to junk food advertising", he added.
"If we don't take bold steps to cut child obesity in London we'll be letting young people down and placing a huge strain on the NHS for years to come".
The Mayor of London has set out a new environment strategy to make the capital the greenest city in the world by 2050.
An Advertising Association spokesperson said the United Kingdom already bans advertising of high fat, sugar and salt (HFSS) food or drink products in all media where under 16s make up more than 25% of the audience. Is that junk food too?
He said he wants to help tackle what he called the ticking time bomb of obesity.
Former mayor of New York, Mike Bloomberg, was among those to praise Khan's move, writing on Twitter that his is a "great example" for other cities seeking to combat the "global epidemic" of obesity.