The origins of the Social Service Employees Union Local 371 can be traced to the 1930s when the State, County and Municipal Workers (SCMWA) served as the nucleus of organizing efforts among city employees for the Congress of Industrial Organizations (CIO). In 1955 when the AFL and CIO merged, their local affiliates in the Welfare Department joined to form Local 371 within District Council (DC) 37 of the American Federation of State, County, and Municipal Employees (AFSCME), AFL-CIO.

Local 371 from its beginning contained seeds of discord despite the apparent apathy of welfare workers toward the state of their union. By the late 1950s a group from the Brownsville Welfare Center located in a poor section of Brooklyn, mainly social investigators, formed the nucleus of a reform movement under the leadership of Sam Podell. Increasingly heavy caseloads and continued low salaries provided two major slogans around which the Podell group rallied the support of caseworkers, most of whom were still not members of Local 371.

The real break-through for the reform movement came in April 1961, when the New York City Department of Labor was involved in a simmering conflict with Jerry Wurf, executive director of DC 37, the parent body of Local 371. Podell, supported by Joe Tepedino (Borough Hall) and Judith Mage (Amsterdam Welfare Center), took advantage of the situation to file a petition for recognition of the Social Service Employees (SSE). The Labor Department, seeking to embarrass Wurf and DC 37, granted the Podell group a charter. Thus the SSEU was born. The SSEU focused organizing efforts on welfare centers and case workers, both of which had hitherto been neglected by Local 371. In a representation election held in October 1964, the SSEU easily defeated Local 371 and secured collective bargaining rights for the civil service titles of case worker, home economist, homemaker and children's counselor.

Close on the heels of the election, SSEU began negotiations for a new contract. These negotiations continued without success, and on December 31, 1964 SSEU members voted enthusiastically for a strike. Under intense pressure from Jerry Wurf, Local 371 also supported the strike. On January 4, 1965, eight-thousand Welfare Department employees went on strike. The strike took a turn for the worse in late January when 19 union leaders were jailed. On January 31, 1965, the strike ended with an agreement to set up a five-person panel chaired by Dean Schottland of Brandeis University's School of Social Work. In June the city and the union signed a contract largely based on the Schottland Committee's recommendations which included most of the original demands made by the SSEU.

In 1966, the SSEU's contract demands included salary increases, promotional opportunities, lower caseloads and better working conditions. The chief bone of contention was the scope of collective bargaining. The SSEU reasserted its 1965 position that any issue should be open to bargaining, while the city wanted to limit areas open to collective bargaining. With negotiations in a deadlock, SSEU members voted to sit-in at their work locations beginning on June 19, 1966. The city retaliated with a lock-out lasting nearly six weeks. Many observers have concluded that the strike was preordained to fail, and their prediction proved to be correct.

After 1967, structural changes in the welfare system and collective bargaining threatened the survival of SSEU as an independent labor union. In April 1968, Judy Mage was replaced by Marty Morgenstern as president. In the ensuing months affiliation with AFSCME Local 371 and DC 37 became the primary concern of the SSEU. The militants campaigned against the move and secured the defeat of the merger referendum on June 28, 1968. But Morgenstern and his supporters continued to argue so vigorously for the merger that the issue won overwhelming support when resubmitted in January 1969. In June representatives of the SSEU and Local 371 formally signed the agreement establishing SSEU, Local 371 of DC 37, AFSCME, AFL-CIO.

SSEU Local 371 is governed by its president, executive vice-president, secretary-treasurer and five vice-presidents elected for two-year terms by the membership at large. The executive committee consisting of all eight top officers and eight delegates elected annually by the Delegates Assembly is entrusted with the functions of policy-making and administrative oversight. The Delegates Assembly is composed of representatives elected proportionally from all work locations on an annual basis. Chapters are organized partially by title and partially by agency and can provide the focus for concerted action by members who work for the same department, agency or bureau regardless of their work location. Although each chapter enjoys a wide range of autonomy on matters pertaining to its members, its decisions are subject to review by the Executive Committee, Delegates Assembly and quarterly membership meetings.

During the 1970s SSEU Local 371 experienced both expansion and change. On the one hand, the union broadened the scope of its recruitment to include more than 100 job titles scattered throughout various city agencies. At the same time the number of caseworkers, who constituted the core of the union, declined sharply because of changes in the welfare structure, automation and the ever-present high turnover. Still, case-workers remained the single largest group of members and continued to hold the top offices of the union. Expansion brought an influx of lower-paid non-professional African-American and Spanish-speaking workers. Gearing its strategies to a more diverse membership, SSEU Local 371 refocused of its collective bargaining concerns, balancing the bread-and-butter concerns of an era of fiscal crisis with the preservation of its progressive, client-oriented traditions.

A new era in the history of the SSEU was ushered in by the election of Charles Ensley as president in 1982. Ensley, an active participant in the civil rights movement of the 1960s, worked as a New York City caseworker after graduating from Howard University, served in the military and returned to the union as a grievance representative. As president he has doubled the membership of SSEU, has fought to improve conditions in homeless shelters, to strengthen services for at-risk children and, in general, to expand services for the poor while improving working conditions for his members. He was on the front lines of protest against apartheid, as well as working to develop special programs for Latino members, and by offering the union's support to politically progressive candidates, Ensley has kept the tradition of the SSEU as a fighting force for social justice alive.