Since 1199 was a "left-led" union, its leadership was investigated by the House Un-American Activities Committee in 1948 for Communist "infiltration". 1199 was a tiny local at the time, however, and during the expulsions of large left-led unions from the CIO in the 1940s, 1199 as a local eventually found shelter under the auspices of the Retail, Wholesale, and Department Store Union. In the late 1950s, the drugstore-based union launched large-scale organizing drives at voluntary hospitals in New York, mobilizing a heavily Black and Puerto Rican workforce in the first flush of the postwar Civil Rights Movement. Martin Luther King famously described 1199 as "my favorite union," and his widow Coretta Scott King became the honorary chair of 1199's organizing campaigns as it sought to expand outside of New York City beginning the late 1960s.
The union's first campaign outside of New York City was the formation of District 1199B in Columbia, South Carolina in 1969. The union led a strike there that never led to a contract, but had success in creating new 1199 districts in Upstate New York, Connecticut, Rhode Island, Philadelphia, elsewhere in Pennsylvania, West Virginia, Kentucky, Ohio, and elsewhere.
Serious faction fights broke out within the flagship New York local and among other 1199 locals after the retirement of the union's original leadership. 1199 eventually left the RWDSU to form a short-lived National Union of Hospital and Health Care Employees during the 1980s, but its constituent locals soon thereafter sought mergers with other unions. Most 1199 locals joined the Service Employees International Union, with 1199C in Philadelphia being the largest 1199 local to join the American Federation of State, County and Municipal Employees. The large flagship New York local remained independent until joining SEIU in 1998.