1974: EMA (Environmental Management Agency) formed and included the following groups:

• Road Department
• OC Parks District
• Flood Control District
• Planning Department

1976: GSA (General Services Agency) formed by a split of Public Facilities from EMA.

1991: Agricultural Commissioner added to EMA

1992/1993: Housing Department added to EMA

1997: EMA and GSA combine to form three new departments including PRFD, PDSD and Housing.

PFRD (Public Facilities and Resources Department)

• Road
• Flood
• Harbors, Beaches and Parks
• Geomatics (Survey)
• Engineering & Permit Services
• Internal Services (added a couple years later)
• Agricultural Commissioner
• Administration
• Watershed (added a couple years later)

PDSD (Planning and Development Services Department)

• Planning
• Building and Safety

Housing Department

2004: RDMD (Resources and Development Management Department) formed by merger of PFRD and PDSD.

The agency headquarters building at 300 N. Flower St. in Santa Ana is one of two Hutton Towers constructed in 1993. The building is named for EMA founding director and engineering pioneer Herbert George Osborne, known as the "father of the Santa Ana River Flood Control Project" for his many significant contributions to planning, public works, and building projects. Osborne began his County career in 1950 as a civil engineer in the Orange County Flood Control District, and held a number of leadership positions before retiring in 1980.