The county had already placed strict restrictions on medical marijuana dispensaries, but officials pointed to the proliferation of shops operating illegally as proof of the need for more regulation.
On Tuesday, the board voted 4 to 1 to approve the ban, with Supervisor Zev Yaroslavsky voting in opposition.
"A complete ban is not going to achieve the objectives that all of us want to achieve with some of these out-of-control illegal dispensaries that have popped up in unincorporated areas," he said. "It's the illegal ones that don't come in for permits that are creating a lot of the problems."
In response to a separate motion from Yaroslavsky, the board voted unanimously to direct county officials to take aggressive action against illegal clinics, including levying a fine of $1,000 per day.
In Glendale, city officials have held off on establishing regulations for the dispensaries, instead enacting a moratorium to give city attorneys more time to analyze the complicated, ever-changing legal landscape.
The county's new ban includes a provision that allows for a return to existing regulations if the California Supreme Court rules that outright bans are unconstitutional.
Howard Hakes, president of the Crescenta Valley Drug and Alcohol Prevention Coalition, said he was glad to hear of the ban, citing stories of recreational users easily attaining medical marijuana prescriptions.
"It's closing another loophole where the drugs were coming from," he said. "I think it's great the supervisors saw that."
Crescenta Valley Town Council President Cheryl Davis said most residents she's spoken with support a ban.
"They want the ban because they feel that if there is a need to get medical marijuana there are legal dispensaries near us," she said.
At Tuesday's meeting, many dispensary operators and medical marijuana patients spoke out against the ban, which they said would provide unnecessary hardship for legal users.
"If you ban, it would take people such as myself out of the loop of providing medicines for chronically ill patients," said Sue Taylor, president of the nonprofit cooperative ICann Health Center. "The ban would do nothing for these pot shops — unregulated clubs that are showing up everywhere."