As part of a deal rubberstamped last month, Ambition, which has six members of staff, has become a subsidiary of UK Youth, which has 90 members of staff, in a move that the two charities said will strengthen the voice and quality of the youth work sector.
Anna Smee, chief executive of UK Youth, told CYP Now that talks between the two organisations have been happening on and off since the 1940s but the time is now right to make the move.
"The will has been there on both sides for a long time but we often had situations where it would work better for one organisation or the other," she said.
"But both organisations were in a good position over the last 12 months with everyone on the same page so we just seized the opportunity."
"We feel we are much more credible now as the one leading organisation that works across non-uniformed and, to some extent, uniformed youth organisations."
Emma Revie, chief executive of Ambition, said coming together strengthens both organisations.
"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."
The two charities will continue to exist as separate legal entities and brands but several Ambition trustees have stepped aside and been replaced by UK Youth trustees.
For the next year the two organisations intend to continue with separate offers to the youth organisations that form their membership but are now beginning a consultation that will inform the introduction of a unified membership offer in April 2018.
Smee said the union of the two organisations also strengths their finances: "UK Youth is already in a cash-rich position and Ambition brings buildings so it will lead to a good combined balance sheet."
The process of integrating staff teams is also in its early stages.
"The legal merger happened with a stroke of a pen but the reality of integrating organisations takes place over time so Anna and I are now working on that integration plan," said Revie.
The move has been welcomed by several youth organisations.
Neil Martin, chief executive of the Jewish Lads' & Girls' Brigade, said: "In these unsettling times we applaud Ambition and UK Youth for taking this bold step to create an even more united and stronger voice for the entire youth sector."
Tracy Clarke, chief executive of Young Gloucestershire, added: "I believe they will be stronger, have clearer strategic direction and provide better value for money. I am not alone in the belief that the way to realise the best future in our sector is through more collaboration of this kind."
The move comes just two years after Ambition, which was known as Clubs for Young People until 2012, merged with the Confederation of Heads of Young People's Services, the organisation for local authority youth service leaders. Ambition also merged with the now defunct National Council for Voluntary Youth Services last year.
It also follows last year's abortive attempt by UK Youth and the National Youth Agency (NYA) to merge their back office functions.
Smee said the UK Youth group has no further merger plans but that could change. "In regards to the NYA, the NYA and UK Youth would have loved to have merged," she said. "Unfortunately we were unable to make that happen but we continue to work closely with them."
Youth minister Tracy Crouch said: "UK Youth and Ambition have both done fantastic work supporting young people across the country and I am confident that this partnership will only strengthen their offering.
"Together I'm sure they will continue to lead the way championing youth voices, and supporting innovation and partnerships."