[18] In 1990, Mendelson resigned from the D.C. Democratic State Committee to work for the reelection campaign of Jim Nathanson, member of the Council of the District of Columbia representing Ward 3.

"[24] In 1992, Mayor Sharon Pratt Kelly's task force on the homeless made a recommendation that shelter beds should be located equally throughout the District.

[6] Mendelson criticized incumbent Harold Brazil for conspicuously leaving a Council meeting just before a vote on whether to reduce pension benefits for newly hired police officers, firefighters, and teachers.

[34] "These end runs, so to speak, in a way empower Barry, because they get him off the hook and play into the voters' reliance on other people to solve our problems.

[35] Mendelson criticized the Department of Recreation for not informing residents ahead of time and for not replacing the equipment immediately.

[38] When the Council unanimously passed a bill to reform the regulatory process, Mendelson said it was hastily written and had not been opened to enough public opinion.

[39] While Council member David Catania admitted the bill was not perfect, he also took issue with Mendelson's approach, saying it undermined District home rule.

[39] In 1998, the Financial Control Board considered repealing a law that required environmental impact statements for private projects costing more than $1 million.

[43] Mendelson said the District should be proactive about recruiting developers and businesses, and he thought he could play a role in doing so as a member of the Council.

[48] The editorial board of The Washington Post endorsed independent candidate Beverly Wilbourn and Republican incumbent David Catania in the general election.

[54] The new bill would reduce monetary penalties for noncompliance, and ban non-compliant companies from doing business with the District for three years.

[55] Mendelson was opposed to Mayor Anthony Williams' proposal to have a children's theme park on islands in the Anacostia River.

In 2011 he supported the effort to establish an elected attorney general[58][59] and pushed for an independent Forensic Science Crime Lab.

[3] From a regional standpoint, Mendelson worked with his counterparts in Montgomery and Prince George's Counties to increase the minimum wage.

[67][68] Mendelson has received national attention for his work to bring Budget Autonomy for the District,[69][70] the city's laws on gun control[71] and same-sex marriage legalization.

[74][75] Mendelson ran for a third term, facing progressive challenger Ed Lazere, and won renomination with 63% of the vote.

[77] In 2019, Mendelson proposed changes that would significantly limit the ability for the public to access government records through the Freedom of Information Act.

Mendelson claimed that the changes were necessary to limit frivolous requests while activists insisted that the changes would prevent the public from uncovering fraud, waste, and abuses.

[78] Mendelson was an active proponent for a no-bid, sole-source contract to launch the District's sports betting program.

[79] After the Council narrowly approved the bid, it was discovered that the main local partner had no employees and was led by a Maryland resident.

Among the many additional regulations surrounding hospitality venue operations, this bill formed the Tipped Workers Coordinating Council within the DC government to oversee the implementation of the law and act as a formal body to make policy recommendations to the appropriate District agency.