Choice Support, which supports people with learning disabilities across England, and mcch, which provides support and housing for people with learning disabilities and mental health needs in London and south-east England, said in a statement that the merger came about to create a major new provider in social care.
The merger will also help the organisations, which employ a total of about 3,000 people, to provide better services and weather any financial challenges in the social care sector, the charities said.
The charities said there were no plans to close any services or offices as a result of the merger, and no front-line staff would lose their jobs as a result.
But a spokesman for Choice Support said the organisations would begin creating back-office functions for PSG in the next few months, which might result in some redundancies among back-office staff.
According to the Charity Commission website, mcch employs 1,653 staff, and Choice Support employs 1,315.
Sarah Maguire, currently managing director at Choice Support, will be the chief executive of PSG after an application and interview process that was open to senior staff from both charities.
The current chief executive of Choice Support, Steven Rose, will work as chief development and innovation officer across both charities, and David Hall, interim chief executive of mcch, is retiring at the end of this year.
The board of PSG will consist of five trustees from each of the charities, with Oliver Mills from mcch becoming chair.
Choice Support had an income of £36m for the year to 31 March 2016, and mcch had an income of £34m for the same year. Both charities are financially solvent and financially viable, the statement from the charities said.
The merger is expected to be completed in the next 18 months.
Maguire said: "Both mcch and Choice Support believe that merging will create a stronger charity that can offer enhanced services to beneficiaries. We’ll become a major new provider in social care, offering support and housing expertise across the country. This is a great opportunity for us to redefine the way we deliver social care.
"We’ll be continuing all the good things that both organisations do currently, while exploring new ways of collaborating with families and commissioners. We’ll share best practice from both organisations and encourage more innovation. In time we’ll be able to offer a broader range of services to people we support. It’s an exciting development."
Mills said: "Social care is facing unprecedented financial challenges. By merging our two charities, which share the same values and strong track records in supporting people with learning disabilities, autism or mental health needs to live independent lives, we are creating a stronger organisation better equipped to face these challenges."