The merged charity will temporarily be known as Rainer Crime Concern, but a staff consultation is expected to produce a new name before the end of the year.
Joyce Moseley, chief executive of Rainer, will take the helm of the new organisation. Clare Checksfield, chief executive of Crime Concern, is expected to leave when the merger is officially completed.
Checksfield said she had been unaware of any merger plans when she joined Crime Concern last September, but insisted she was happy with the situation. She said she would be looking for a new, challenging role in the voluntary sector.
The charities said they were merging for strategic reasons and not because of any financial concerns. Only a handful of staff redundancies are expected, and staff involved with projects in the field should be unaffected.
Rainer and Crime Concern currently work in about 150 deprived areas across England and Wales, largely carrying out central and local government contracts.
Both charities have similar sources of funding, mostly from grant makers and government bodies. However, Moseley has said in the past that she wanted to increase the amount of money raised from donations, and promised that the new organisation would continue this policy.
The new charity's head offices will be in Old Street, London. Staff from Rainer will move to the new office next week, but Crime Concern staff will remain in their current office.
"What's been surprising so far is how easy it has been," said Moseley. "Trustees of the two organisations met in September last year to discuss the merger, and the respective management teams met not long after.
"Everyone saw the sense in it. Everyone has come together in harmony. There has been no standing on ego at all."