Wandsworth Council said it has struck a deal with 4Children and The Prince's Trust to deliver services in partnership.
The council first announced its intention to outsource youth services in January last year stating at the time that the contract would be worth between £10m and £20m over a six-year period.
It covers the provision of youth work services, youth crime interventions and gang prevention work.
"4Children and The Prince's Trust will bring a wealth of expertise and new ideas to our youth services, ensuring local young people continue to benefit from some of the very best facilities in the country and a superb choice of activities, support and learning opportunities," a Wandsworth Council spokesman said.
“Like every council, we have less money to spend each year and have to make savings while preserving frontline services.
"Harnessing the innovation and skills that exist within organisations like 4Children is central to our approach.”
The council said the decision to outsource youth services was part of an overall strategy to run a “leaner and more cost-effective town hall”.
A survey conducted before the decision to outsource provision was taken found that residents "did not care" who delivered the local services as long as they were to a good standard.
Imelda Redmond, chief executive of 4Children, said: “Together with the programmes and experience brought by The Prince’s Trust, we will provide a safe space where young people can socialise and take part in a wide variety of activities and opportunities.
“Not only are good youth services vital for young people, they are incredibly important for the wider community too – and can have a real impact on community cohesion and crime prevention.”
