The charities said the move would involve Angelus, which works to raise awareness of the risks of so-called legal highs, operating under the Mentor name and branding from 1 October.
Angelus’ four staff members will move into the offices of Mentor, which focuses on drug prevention among young people and employs 21 people.
Michael O’Toole, chief executive of Mentor, will lead the new organisation, with Jan King, chief executive of Angelus, leaving as part of the merger process, although she will stay to help with the handover.
O’Toole told Third Sector that the impetus behind the merger was a desire to enhance the work of both charities and have greater impact on more people.
He said both charities’ finances were strong and that money was not a factor in the decision to merge the two organisations.
According to the Charity Commission website, Angelus Foundation had an income of £283,109 and expenditure of £300,006 in the year to 31 March 2015.
The Charity Commission website also shows that Mentor had an income of £723,902 and expenditure of £694,245 for the same financial year.
No decision has yet been taken on the long-term structure of the charity’s board following the merger.
O’Toole said: "This merger is a great match of expertise – it is going to give fresh impetus to the prevention agenda. Mentor and Angelus working together will certainly enhance our capability to deliver on the full range of issues affecting young people.
"Angelus has shown it is the lead voice in educating young people and the public about the new phenomenon of new psychoactives. Together we will be a stronger force to ensure we build even more young people's resilience to the wide range of pressures they face."
King said the merger was the best option to increase the charity’s work with young people.
"There is no organisation in the sector which is more respected than Mentor and they have a very strong track record of delivering high quality prevention programmes," she said.
"We look forward to reaching more young people and enabling them and their parents to be better equipped to cope with the risks that drugs present."