The girls, 10 and 11 years old, ate the cookie and were hospitalized on Monday for marijuana poisoning.
The Covina Police Department said the mother, 33-year-old Veronica Sylvester, admitted that she knew the cookie contained pot.
At first, Sylvester said the girls brought the cookie home from school. Later she said a friend gave her the cookie and she did not intend for her daughters to eat it.
Family members took the girls to an area hospital after they complained of feeling ill. Medical tests showed traces of marijuana in their systems.
Sylvester was arrested for child endangerment on Tuesday when she owned up to leaving the marijuana-laced cookie on the kitchen counter.
Covina Police Sgt. Dave Foster said Sylvester did not have a medical marijuana prescription.
"[Police] learned that mom had obtained the cookie from an unknown friend for an unspecified medical condition that she has, and that she had left it in the kitchen of the home," said Foster.
Sylvester was held at the Century Regional Detention Facility in Lynwood. Bail was set at $100,000.
Police said Sylvester's daughters are doing fine and were staying with family members.