In 1953, faced with recurring droughts, an expanding population and economy that would soon exhaust local drinking water supplies, voters in southern Ventura County united to form the Calleguas Municipal Water District for the purpose of providing the region with a reliable supply of high quality supplemental water.  Finding a dependable water source for a semiarid region such as Ventura County was no simple feat: adequate rainfall is unreliable, groundwater often brackish and unsuitable for both urban and agricultural uses, and rivers are seasonal.
Following a thorough evaluation of potential water sources, the District turned to the snow covered peaks of the Sierras and the State Water Project's system of canals, reservoirs and pumping plants that convey water to many regions throughout the state.

To secure water from the state system, voters approved another ballot measure in 1960 so that Calleguas could join the Metropolitan Water District of Southern California, a state water contractor.

Shortly after becoming a member agency of Metropolitan, Calleguas assumed the task of building a pipeline network to bring imported water into Ventura County. 
It cost $22 million to construct the more than one-mile-long Santa Susana Tunnel through the mountains east of Simi Valley, along with the necessary pipelines to connect to Metropolitan's system in Los Angeles County.
In 1965 Calleguas completed Lake Bard, a surface water reservoir, to store excess water for use to meet peak summer and emergency demands.
Following the completion of the first phase of the State Water Project in the early 1970s the pipelines were linked to Metropolitan's Jensen Filtration Plant in Granada Hills.

In 1991, the State of California imposed new water quality regulations requiring that previously purified water be treated again if it had been stored in an uncovered reservoir.  To comply with the new standard, Calleguas designed a treatment system for water stored in Lake Bard that not only meets all federal and state regulations, but surpasses them.  In 1995 Calleguas completed a $23 million state-of-the-art treatment plant that disinfects water with ozone gas in place of traditional chlorine treatment.  Calleguas finished an expansion of the plant in 2005 bringing its present capacity to 65 million gallons per day.

Communities served include the cities of Oxnard, Port Hueneme, Camarillo, Thousand Oaks, Moorpark, Simi Valley, and the unincorporated areas of Oak Park, Santa Rosa Valley, Bell Canyon, Lake Sherwood, Somis, Camarillo Estates, Camarillo Heights and Naval Base Ventura County.  The district serves an area of approximately 365 square miles.

Over the last five decades, the number of residents in the district's service area has more than quadrupled from 138,000 in 1964 to an estimated 615,000 in 2010, roughly 75 percent of Ventura County's population.

Rapid population and economic growth has placed increasing demands on the District resulting in an increase in annual deliveries from 9,000 acre feet to in excess of 100,000 acre feet over the same period. Calleguas' member purveyors receive water through 140 miles of large-diameter pipeline operated and maintained by the District Those purveyors distribute the water for ultimate delivery to residents and businesses.

It was the foresignt of members of Calleguas' original Board of Directors that led to the creation of the District over fifty years ago.  Today Calleguas continues to look ahead to meet the needs of its service area for generations to come.  Calleguas continues to develop groundwater storage in the Las Posas Groundwater Basin.  The basin is an 18-mile-long geological formation of sand, gravel and clay layers where water is naturally retained.  It is estimated that up to 100,000 acre-feet of water, roughly ten times the capacity of Lake Bard, can be held in reserve for later use.  If the state supply is reduced or disrupted, stored water will be recovered through a system of groundwater wells and treated prior to delivery to local water users. 

In addition, the District has begun construction of the Regional Salinity Management Pipeline (SMP). Salt levels are increasing in the Calleguas Creek Watershed's groundwater and surface water, rendering Calleguas Creek and many of its tributaries unsuitable for a variety of uses.  The SMP will collect and convey brine concentrate from new groundwater treatment plants to be sited throughout the watershed, as well as surplus high quality recycled water, for use in wetlands restoration projects, irrigation of salt tolerant crops or discharge to the ocean through a newly constructed outfall.  Allowing for the development of additional groundwater resources, the SMP will greatly reduce the region's dependence on imported State water and enhance area surface and groundwater resources.