Williams was elected to her first term as state representative of Illinois' 11th District in 2010 to replace John Fritchey who chose to run for a seat on the Cook County Board of Commissioners. Williams was supported by both of Chicago's major newspapers. The Chicago Tribune noted in its endorsement that "It's good to see that she believes party leaders shouldn't be exempt from the new campaign contribution caps imposed on everyone else."[4] The Chicago Sun-Times called her "bright and conscientious" and noted her work for Attorney General Lisa Madigan and as a staff attorney for the state House.[5] Williams focused her campaign on protecting women and children from sex offenders, fighting for greater access to affordable healthcare, and making government open and accountable to the people.[6] Her Committee assignments include Judiciary I - Civil Law; Consumer Protection; Adoption Reform; Business Occupational Licenses; Environmental Health; and Government Operations Subcommittee.[1]
Williams won reelection to a second term in 2012. She ran unopposed in the March 20 Democratic primary and was unopposed in the general election, which took place on November 6, 2012.[7][8][9] Williams was re-elected to a third term in November 2014.[10]
In 2018, J. B. Pritzker appointed Williams to Powering Illinois’ Future transition committee, which is responsible for infrastructure and clean energy policies.[11]
^Demarest, Erica (December 7, 2011). "State Rep. Ann Williams talks adoption, LGBT outreach". Windy City Times. Archived from the original on January 20, 2012. Retrieved December 7, 2011. A couple issues that always come up—unfortunately, even those it's 2011—are things like [abortion] choice and civil unions. Those are a priority for a lot of us in this region. I wasn't there to take the vote for civil unions. [Voting occurred before Williams took office.] It would've been a real highlight and privilege to get to vote for that, but I hope to be around for the vote that will provide full marriage equality. I hope that comes sooner rather than later for the families in my community.