She came equipped with a folder filled with info on the Illinois elections, including notes on judges and other down ticket contests.
"She takes this very seriously," says a White House staffer.
Obama showed her drivers license before taking her ballot and presumably voting for Alexi Giannoulias and Governor Pat Quinn in the state's two biggest races.
The woman voting next to her asked if she could snap a cell phone picture and Mrs Obama obliged. As she handed in her ballot she said "make sure everyone is voting early."
She told supporters:
From here she heads to Colorado for senate candidate Michael Bennett.