Marisela Barraza was arrested on Monday after her sister noticed the girl was speaking slowly and her eyelids were drooping.
She brought her to the hospital and blood tests confirmed there was marijuana in her system. The 19-year-old told police her daughter ate the pot brownie before she could stop her.
She also told them it was no big deal and that marijuana should be legalized.
If convicted, Barraza faces six months in jail.