Sonoma County was one of the original counties formed in 1850. Its northern boundary followed the east side of the Russian River to its headwaters north of Ukiah. Mendocino County covered the area west of the river to the coastline and north to Trinity County. Mendocino was only sparsely populated and was not fully organized, so it was attached to Sonoma County "for judicial purposes.1" In 1855 Sonoma and Mendocino exchanged land, and Sonoma’s northern border went from around Fort Ross on the coast up to just south of Ukiah. It wasn’t until 1859 that Mendocino County became fully organized, detached from Sonoma County, and the boundary lines became close to what they are today. Sonoma County’s eastern and southern boundaries also changed through the years, but the differences were not quite so dramatic.
So if you are looking for someone who lived near Healdsburg in the 1850 census, you may find them in Mendocino County!
For more detailed information, take a look at the Newberry Library's websites:
California: Individual County Chronologies (scroll down the alphabetical list)
1 California: Individual County Chronologies, Atlas of Historical County Boundaries, ed. by John H. Long, Chicago: The Newberry Library, 2011 (https://publications.newberry.org/ahcbp/documents/CA_Individual_County_Chronologies.htm#Individual_County_Chronologies : accessed 6 January 2019)