The charities will combine to form a new charity, called the Helplines Partnership, under which name both charities will continue to exist.
It will be headed by Theodore Spyrou, the chief executive of the Mental Health Helplines Partnership. Rekha Wadhwani, chief executive of the Helplines Association, will leave her post.
A spokesman for the partnership said the merger process was likely to be concluded after meetings between both sets of staff before the end of this month.
The Mental Health Helplines Partnership provides telephone support services to those with mental health issues and acts as an umbrella organisation for groups such as OCD Action, Anxiety UK and Combat Stress. The charity employs six staff and had a turnover of £136,577 in 2010/11, a spokesman for the partnership said.
The Helplines Association, which has two employees, promotes good practice through helpline services in the UK and Ireland. It had an income of £618,588 in 2010/11, according to figures on the Charity Commission website.
The board of the new charity will consist of the existing trustees of the Mental Health Helplines Partnership plus the chair, vice-chair and treasurer of the Helplines Association.
A joint statement from Peter Calderbank, chair of the Helplines Association, and Liz Felton, chair of the Mental Health Helplines Partnership, said a merger would benefit members, beneficiaries and the sector.
"The Helplines Association and the Mental Health Helplines Partnership have had an excellent working relationship over many years," they said. "A merged organisation will offer members easier access to an expanded range of services and enable the combined organisation to deliver them more effectively."