At the signing on Wednesday, Quinn was joined by dozens of elected officials, including Secretary of State Jesse White, Attorney General Lisa Madigan, Cook County President Toni Preckwinkle, and Comptroller Judy Baar Topinka.
"Marriage equality is about two American principles: fairness and freedom," Madigan said.
"It takes both parties to make something happen. And when we work together, look at what we can do," Topinka said. She also offered to be a flower girl, free of charge.
"As a history teacher, I believe marriage equality is the civil rights issue of our time," Preckwinkle said.
"There is no straight or gay marriage. From now on there is only marriage in Illinois," Chicago Mayor Rahm Emanuel said. "To be a forward moving state, you cannot have backward-looking laws."