In 2011, Sims announced his intentions to run for representative of the 182nd Legislative District in the Pennsylvania House of Representatives. Sims defeated Babette Josephs, a 28-year incumbent, in the 2012 Democratic primary. He did not face a Republican challenger in the November general election and was elected. Sims was the first openly gay person elected to the Pennsylvania General Assembly. Although he was not sworn in until January 1, 2013, because Pennsylvania state representatives' term of service and legislative duties officially begin on the first day of December following their election, Sims shares the designation of being its first openly gay member with Rep. Mike Fleck (R–Huntingdon), who came out in a newspaper article published later that day. In June 2013, after the Defense of Marriage Act had been ruled unconstitutional by the Supreme Court, Sims tried to make a speech in the Pennsylvania House supporting the decision, but was blocked by Daryl Metcalfe, among others, who called Sims' comments "open rebellion against God's law." Sims made national news on October 3, 2013, when he and fellow Democratic Rep. Steve McCarter introduced legislation to legalize same-sex marriage in Pennsylvania. Sims has also introduced a bill with fellow Democratic State Representative Erin Molchany to help reduce and eliminate the gender gap in rate of pay as well as legislation to ban the practice of conversion therapy with Rep. Gerald Mullery. Sims has also made efforts to work with federal legislators on issues of LGBT civil rights. On March 28, 2013, Sims penned an open letter to U.S. Senator and fellow Pennsylvania Democrat Bob Casey Jr. urging him to come out publicly in support of same-sex marriage. This, combined with many other calls, ultimately resulted in the senator voicing his support for the measure. Senator Pat Toomey (R-PA) also chose to vote for the Employment Non-Discrimination Act (ENDA) in the U.S. Senate after Sims and a number of other activists wrote to him on the matter. On November 11, 2013, Sims teamed with Republican State Representative Bryan Cutler to introduce a bill to replace Pennsylvania's system of electing judges with a merit-based system, which did not receive debate in the PA House. Sims served on the House Commerce, Game and Fish, Human Services, State Government, and Tourism and Recreation Committees. Sims served as Democratic Chair of the Human Services Subcommittee on Mental Health. Sims has served as the prime sponsor of