Ambition, a membership body for organisations that work with young people in the UK, has become a subsidiary of UK Youth, which provides services to young people through a network of local youth organisations across the UK.
The charities said they would be able to achieve more by working together and would be better placed to respond to the challenges faced by young people today.
A spokeswoman for the charities said both organisations would keep their individual names and brands, although members of both charities will be consulted on future branding and priorities for 2018.
The charities will continue to have separate trustee boards, although five UK Youth trustees will join the Ambition board to take a majority position.
The spokeswoman said Anna Smee and Emma Revie, the chief executives of UK Youth and Ambition respectively, began discussing a merger shortly after Revie took up her post last September.
Smee will continue as chief executive of UK Youth, which has 90 employees, and Revie will remain chief executive of Ambition, which has six members of staff.
The spokeswoman said there had been no redundancies as a result of the merger, which was completed on 15 September.
Ambition and UK Youth will maintain their offices in Vauxhall and Highbury respectively, both in London, although it was likely that staff from both organisations would work across both sites, the spokeswoman said.
Both organisations have experienced falling income in recent years, with UK Youth’s falling from £8.6m in the year to 31 March 2015 to £5.3m a year later, while Ambition’s was down from £2.1m to £1.3m over the same period.
But the spokeswoman said both charities had reported healthy finances with strong reserves in recent years. "While this merger might allow for cost savings through shared services, it was inspired by a joint mission to provide all young people with access to high-quality and appropriate youth services in their local community," she said. "Both organisations bring significant financial assets to the partnership."
Records showed that the possibility of a merger was first discussed between the two organisations in 1945, the spokeswoman said.
Smee said the charities had a shared commitment to empowering young people and working in partnership with local, regional and national youth organisations to deliver high-quality services.
"Together we will be able to amplify the voices of more young people and respond to their needs by forging wider partnerships to deliver joined-up services in local communities," she said.
Revie said: "We’re passionate about supporting youth organisations to provide high-quality services for young people, whatever their backgrounds or circumstances. By joining forces with UK Youth, I’m confident we have the potential to be greater together than the sum of our parts, and I’m excited to see what we can achieve."