The law takes effect in the 2014-2015 school year. It lowers the compulsory age from 7 to 6.
Quinn signed the bill Sunday at a school on Chicago's West Side. Monday marks the first day of school for hundreds of thousands of students in Chicago Public Schools.
Quinn says the bill will help students get an early start on their education.
The bill was pushed to lower absenteeism in elementary school. About half of the states have a compulsory age of 6.
Opponents, including Republicans, had questioned the cost.