The Association of Community Human Service Agencies (ACHSA) was formed in February 2001 upon the merger of two Los Angeles County associations representing agencies serving persons in the County mental health and child welfare/juvenile justice systems – the Association of Community Mental Health Agencies (ACMHA) and the Association of Children's Services Agencies (ACSA).

ACMHA was formed in the early 1980s to represent the interests of private nonprofit community mental health agencies and the children, adults, and older adults with mental illness that they serve. The mission of ACMHA was to promote the role of the private nonprofit sector in mental health service delivery and to provide mutual support in pursuit of a more effective community mental health system serving people suffering from mental illness. ACSA, formed in 1955, represented private nonprofit child welfare agencies serving children in the foster care and juvenile justice systems. The mission of ACSA was to represent and advocate for high quality services which support and provide for the safety and health of children and their families.

The merger was the result of a consensus among the two Associations that integrated delivery systems providing a broader base of services are better positioned to respond to the increasingly complex needs of the individuals, families, and communities that the Associations' agencies serve. Member agencies of the two Associations had been moving away from categorical, specialized operations to client-driven continuums of care. Therefore, it only made sense that the Associations serving their Member agencies themselves should similarly consolidate the manner in which they were organized to best meet the needs of increasingly diverse Member agencies and the vulnerable populations that they serve.