YouthNet, which provides online support and advice, and Get Connected, which runs a telephone helpline for people aged under 25, agreed to merge last year in a bid to increase their reach from 1.7 million 16 to 25-year-olds to two million by 2018.
A spokeswoman for the charity said three redundancies had been made since the merger was announced.
Both charities employed 24 people at the time of the merger, meaning The Mix now employs 45 staff.
The spokeswoman said YouthNet and Get Connected would operate as two departments, with the Get Connected part covering the helpline and counselling services and the YouthNet part handling digital services.
The spokeswoman said The Mix had moved all its staff into two premises near Baker Street and on Oxford Street, both in London.
The premises were given as a gift in kind by the firm Dixons Carphone, one of the charity’s biggest supporters, the spokeswoman said.
The chief executive of The Mix is Chris Martin, and a new combined trustee board has been created with Luke Taylor, global chief executive of the technology agency DigitasLBi, as its chair.
"Our new charity brand reflects the importance for us that young people remain at the heart of everything we do," said Martin.
"The Mix was created jointly with young people, our volunteers and staff. It reflects the blend of services we offer young people, delivered on their own terms and through the technology of their choice. Young people today face unprecedented challenges and there has been escalating demand for our services. The Mix will always be there for them, providing help, information and support."
In the last set of accounts for both charities, YouthNet had an annual income of £2.2m, compared with £1.3m for Get Connected.