Recently, Broguiere's Dairy in Montebello (founded by a French immigrant in 1920; still family-owned 99 years later) announced plans to close.
I have sent Broguiere's an interview request and hope they'll have the time and inclination to talk to me (closing down a business is more work than most people realize). In the meantime, let's explore LA County's other French-founded, long-forgotten dairies. (I'm sure I don't need to remind my readers that French cuisine is butter-based, and living in then-remote Southern California didn't necessarily change French Angelenos' culinary preferences all that much.)
Much of the information about these dairies is lost to history; I'm afraid I won't be able to offer as much information as I prefer.
Augustus Ulyard is known to have established a dairy in Cahuenga (somewhere in the Valley) after retiring from baking.
Paul J.M. Molle is known to have been in the dairy business, and likely rented land on Rancho Topanga Malibu Sequit for that very purpose (yup...it's quite possible there were once dairy cows in Malibu).
Jean Sentous' dairy farm stood in the block bordered by Washington, Grand, 21st, and Main (this piece of land would later become Chutes Park). His cattle brand is now in the permanent collection of the Natural History Museum of Los Angeles County.
Francois Pellissier, known for the Pellissier Dairy and Alpine Dairy (he also co-founded the Highland Union Dairy), raised his dairy cows on a ranch straddling Whittier and City of Industry. The family farmhouse stood on Workman Mill Road in what is now Industry, but much of modern-day Whittier was originally the Pellissier family's sizable dairy.
The Didier family also raised dairy cows in the City of Industry area (the Homestead Museum has a surviving Didier Dairy milk bottle in its permanent collection, although it's not on public display).
We're known for LA's earliest commercial wine production. Apparently, we made at least some of the cheese that went with it, too.