Sunnyslope County Water District was founded on December 17, 1954 to provide potable water supplies to San Benito County residents in the area east of the City of Hollister boundary at that time. As Hollister continued to grow and develop, many of these areas were eventually annexed into the city, but still retain water service from Sunnyslope. 

In the early 1970's with the development of the Ridgemark Country Club to the south east of Hollister, Sunnyslope agreed to provide sanitary sewer service in addition to potable water service. This sewer service eventually expanded to include the Quail Hallow and Oak Creek developments in the mid-1990's. 

Sunnyslope maintains a strong relationship with neighboring agencies including San Benito County Water District and the City of Hollister. Significant coordination with these agencies has culminated in various regional plans for water supplies and quality, wastewater treatment and disposal, and long-term partnerships to accomplish many diverse goals.