Sadiq Khan's plans will let young people caught with cannabis off the hook.
He disputed allegations he was seeking to "decriminalise" narcotics in London, claiming he had no ability to do so.
Under a pilot programme, 18 to 24-year-olds caught with "a little amount" of cannabis would not be arrested but instead provided drug education classes.
The mayor's spokeswoman denied the mayor was considering putting ketamine and speed in the scheme.
The pilot would span Lewisham, Bexley, and Greenwich.
Mr Khan, according to his spokeswoman, "firmly believes that drug usage and related crimes are preventable and not inevitable."
"We know we can't simply arrest our way out of the problem, which is why we keep working on programmes that provide young people with help and education, rather than simply sending them through the criminal justice system," he continued.
Although the scheme's funding hasn't been approved yet, it is expected later this month.
The Met Police came under fire over the weekend after publishing a video of police drug testing Londoners on a night out.
On the streets of Shoreditch, cops swabbed hands and searched a man whose face was obscured.
The video attracted hundreds of comments, many questioning the legality of the police action and compared it to the Met's decision not to investigate alleged COVID-rule-breaking parties conducted in Downing Street.
"Bit strange, considering if they had narcotics in their bloodstream, that would be a historic crime," one Twitter user remarked.
David Banks, a media law expert, asked the cops: "Many people wonder what legal authority you had to do this. Describe."
The Met Police released a lengthy statement stating that the drug swabbing was part of a December "week of action" to defend women's safety.
The force said it swabbed people for drugs as a condition of entry to two licenced venues.
"The swabbing was consensual," the Met Police added.
"Refusal does not necessitate a search under S23 of the Misuse of Drugs Act."