"We believe in liberty and equality," Quinn said before signing the bill at a desk once used by Abraham Lincoln.
For the 2,000 or so in attendance, the signing of the bill was personal.
U.S. Marine Brad Setter said he was suicidal while he served from 2007 to 2011 — when "Don't Ask, Don't Tell" was the law of the land.
"It was really tough on my mental life, but the last six months were the hardest," the 24-year-old Setter said. "People found out I was gay, and I began getting death threats."
After hitting bottom, he decided not to give up on life and became an LGBT activist. On Wednesday, he was one of thousands who gathered at the arena to watch Quinn sign the same-sex marriage bill into law.
"Today is a day where we can start moving forward and educating people about gender and sexuality," Setter said.
Among the elected officials attending Wednesday ceremony were Quinn, Mayor Rahm Emanuel, Attorney General Lisa Madigan and Lt. Gov. Sheila Simon.
"It's time to stop planning rallies, start planning weddings," Simon told the crowd before the signing.
The Illinois Senate passed the Religious Freedom and Marriage Fairness Act in February, but the bill stalled in the state House in May. This prompted several summer protests and a "March on Springfield" in late October.
On Nov. 5, both houses of the Illinois General Assembly signed off on the bill.
"Sometimes we walk slowly, but we never walk back," Rep. Greg Harris (D-Chicago), the bill's main sponsor, said Wednesday.
Jim Darby and Patrick Bova, long-time Chicago gay rights activists, made the crowd swoon and cheer as they embraced and shared a kiss on stage.
After 50 years of being together "today is the day we can finally be newlyweds," Bova said.
Until 2012, only six states recognized same-sex marriage: Iowa, Connecticut, New York, Massachusetts, Vermont and New Hampshire. Last November, Maine, Maryland and Washington joined the list.
In 2013, seven more states legalized the unions: California, Delaware, Rhode Island, Minnesota, Hawaii, New Jersey and Illinois.
"There is no straight or gay marriage. From now on there is only marriage in Illinois," Emanuel told the crowd to cheers Wednesday.
The mood was nothing short of elated as Quinn, surrounded by about a dozen politicians, signed the bill into law. Pens used in the signing were handed out to eager attendees as others posed for pictures and selfies.
"I'm going to tell my nieces and nephews and my kids I was here today," said Stacy Gorgas, a 36-year-old who showed up for the signing early. "This is a product of a lot of people's efforts, not just gay people but straight people as well."
Gorgas and his friend Adam Roker were among the people who showed up at the arena before doors opened.
For Roker, a 27-year-old gay man, the same-sex marriage bill shows how much progress has been made when it comes to equal rights for gays and lesbians.
"In just 10 years, my mom went from being completely against anything to do with my sexuality, as in 'not a fan,' to saying, 'you can't marry a man until I meet him first,'" Roker said.