About 1,000 people crowded into the Chicago Cultural Center to watch Quinn, a Democrat, sign the measure that supporters call a matter of basic fairness and opponents decry as a threat to the sanctity of traditional marriage.
"We believe in civil rights and we believe in civil unions," Quinn said before signing the bill to a roar of cheers and applause.
The law, which takes effect June 1, gives gay and lesbian couples official recognition from the state and many of the rights that accompany traditional marriage, including the power to decide medical treatment for an ailing partner and the right to inherit a partner's property.
Five states already allow civil unions or their equivalent, according to the Human Rights Campaign. Five other states and Washington, D.C., let gay couples marry outright, as do some countries, including Canada, South Africa and the Netherlands.
Illinois law will continue to limit marriage to one man and one woman, and civil unions still are not recognized by the federal government.
Chicago residents Amanda Barlow, 43, and Mimi Reynolds, 47, said they will hold a civil union ceremony this summer. The couple, who have 4- and 5-year-old boys and have been together 14 years, had considered traveling to other states that allowed same-sex civil unions or marriage, "but I kept telling her 'No, it's going to happen here in Illinois,'" Barlow said.
"For us to witness and see it happen and to realize that we're both living our dreams . . . it just solidifies who we are as a family," Barlow said after the bill-signing. "So I'm speechless because I feel like I'm living in a dream come true."
They also appreciate the law's protections: When Barlow was diagnosed two years ago with breast cancer, Reynolds had to produce documents from a lawyer to prove to hospital officials that she had the right to visit Barlow and help make decisions.
Now, "all those who . . . might have to go through this someday won't have to go through a lawyer just to have basic rights," Reynolds said.
Opponents, including some religious and conservative groups, said the law is a step toward legalized same-sex marriage.
"Marriage was not created by man or governments," David E. Smith, executive director of the Illinois Family Institute, said Monday. "It is an institution created by God. Governments merely recognize its nature and importance."
Some hope civil unions are a step toward full marriage for gay and lesbian couples, although sponsors of the civil union bill have said they don't plan to push for legalizing same-sex marriages, which have limited support in the Legislature.
Cardinal Francis George and other Catholic leaders also vigorously fought passage of the law. The measure doesn't require churches to recognize civil unions or perform any kind of ceremony, but critics fear it will lead to other requirements, such as including same-sex couples in adoption programs run by religious groups or granting benefits to employees' partners.
The legislation, sent to Quinn in December, passed 61-52 in the Illinois House and 32-24 in the Senate.
"Illinois is taking an historic step forward in embracing fairness and extending basic dignity to all couples in our state," John Knight, director of the Lesbian, Gay, Bisexual, Transgender Project of the ACLU of Illinois, said in a written statement issued hours before the bill-signing.