Sigcho-Lopez unsuccessfully challenged Danny Solis in the 2015 Chicago 25th Ward aldermanic election.

On April 10, 2019, prior to being sworn in, Sigcho-Lopez joined six other newly elected members of the City Council in protesting against the approval of tax increment financing for the Lincoln Yards and The 78 real estate developments.

On June 12, 2019, Sigcho-Lopez demanded a review of all permits and licenses approved by disgraced former Alderman Danny Solis.

[9] He joined all five other members of the Socialist Caucus in signing a letter to Lightfoot which criticized her budget for "an over-reliance on property taxes" and "regressive funding models" that are "burdensome to our working-class citizens, while giving the wealthy and large corporations a pass.

[11] On March 22, 2024, Sigcho-Lopez spoke at a City Hall rally against the Israel–Hamas war where Marine veteran Zachary Kam had burned a flag that he had flown while on tour in Afghanistan, prompting some of Sigcho-Lopez's City Council colleagues to call for him step down as chairman of the Council's Housing Committee.