Orinda is a city in Contra Costa County, California, United States. The city's population as of the 2020 census is estimated at 19,514 reside…Orinda is a city in Contra Costa County, California, United States. The city's population as of the 2020 census is estimated at 19,514 residents. Orinda is located within four Mexican land grants: Rancho Laguna de los Palos Colorados, Rancho Acalanes, Rancho El Sobrante and Rancho Boca de la Cañada del Pinole. The area was originally rural, mainly known for ranching and summer cabins. The Moraga Adobe was built in 1841, and is the oldest building in the East Bay. In the late 19th century, the land was named by Alice Marsh Cameron, probably in honor of the poet Katherine Philips, who was also known as the "Matchless Orinda".