After running unsuccessfully against Phil Reed for City Council in District 8 in 2003, Mark-Viverito was elected to the position when Reed reached his term limit in 2005. During her first four years in office, Mark-Viverito sponsored and passed several pieces of legislation regarding tenant harassment, building safety, greening buildings, and park conservancies. In January 2009, she criticized the voting record of newly appointed New York SenatorKirsten Gillibrand on immigration. During Mark-Viverito's second term in the Council, she served as chair of the Parks and Recreation Committee and as founding co-chair of the Progressive Caucus. In November 2013, she won re-election to her third term in the Council, and her close ally Bill de Blasio was elected mayor. Soon the New York Daily News cited Mark-Viverito as "the front-runner" for "New York City's second-most powerful political post — Speaker of the City Council." A grassroots effort to boost her Speaker candidacy included social media, fliers, phone banking, and volunteer recruitment. Mark-Viverito was elected City Council Speaker on January 8, 2014, at age 44, becoming the first member of the Council's Black, Latino and Asian Caucus to hold this position. Her first "State of the City" speech emphasized reform of the criminal justice system. In January 2016, Mark-Viverito introduced a collection of eight bills known as the "Criminal Justice Reform Act" to reduce the penalty for acts such as violating park rules, littering and public urination from misdemeanors to the civil process. In addition, the Criminal Justice Reform Act also suspended enforcement for "excessive noise," which resulted in a sudden spike in noise complaints around the five boroughs, according to data compiled by 311, New York City's complaint submission platform. Mark-Viverito sponsored the bills so that young people in communities of color could "fulfill their potential" by incentivizing officers to give verbal warnings and fines but not remove the option of making arrests. In 2016, Mark-Viverito was alleged to have pressured the New York City Housing Authority to fire a black manager of a NYCHA housing project and replace her with a "Spanish Manager". As of 2018, the lawsuit is still pending and the City of New York's motion to dismiss the lawsuit was denied. In 2017, Mark-Viverito declined to boycott the Puerto Rican Day Parade, after organizers decided to honor Oscar López Rivera, a prominently incarcerated member of Fuerzas Armadas de Liberación Nacional Puertorriqueña.
Mark-Viverito campaigned in solidarity with the Occupy Wall Street movement and identified herself as part of the "99%" of income earners despite having a net worth in the millions. She and her family inherited an estimated $6.7 million from her father, a doctor. Prior to her inheritance, she applied for a taxpayer-subsidized loan in 1998 when her property at the time was worth $310,000. As of 2014, the multi-story condominium property was worth around $1,300,000. Mark-Viverito rents out her properties, but did not report her rental income. Her spokesman claimed it to be an honest oversight on her part. In August 2014, Mark-Viverito publicly disclosed that she was infected with the most common STD, the human papillomavirus.
