Daranee, 46, made a series of inflammatory speeches at the red-shirt rallies, notably the demonstration last June at Sanam Luang. Her remarks were against the 2006 coup but laced with offensive references to the monarchy.
The court said in its verdict that Daranee had maliciously offended the monarchy as per Article 112 of the Criminal Code.
She was penalised for three counts of lese majeste, each carrying the punishment for six year imprisonment.
The court found no cause for leniency because the defendant showed no remorse.
About 30 supporters showed up at the verdict session but none dressed in red.