Mack first moved to Oxnard, California, and he worked at the Camarillo State Mental Hospital. From 1964 to 1969, he was the executive director of the Flint Urban League in Flint, Michigan, where he focused on "fair housing and voter registration issues." Mack later returned to California, where he served as the president of the Los Angeles Urban League from 1969 to 2005. He co-founded the Los Angeles Black Leadership Coalition on Education in 1977, and also became vice president of the United Way Corporation of Council Executives. In the aftermath of the 1992 Los Angeles riots, Mack showed President George H.W. Bush around South Los Angeles. At the time, Mack and Mayor Tom Bradley were criticized by young activists for being too removed from the real life experiences of everyday African-Americans; Mack rejected the criticism. Over the next few years, he helped rebuild the area, especially the Crenshaw Boulevard corridor. He became an advocate for neighborliness between blacks and Hispanics in the area. Mack was the president of the board of police commissioners of the Los Angeles Police Department from 2005 to 2007, and as a member through 2013. Under his leadership, the commission became the "driving force" behind several LAPD improvements, including video cameras in squad cars to increase accountability among patrol officers, and the department achieving full compliance with the federal consent decree that had been in place since 2001. He subsequently served on the Los Angeles City Planning Commission. In 2018, Mack supported the nomination of Michel Moore as the new Chief of the Los Angeles Police Department. In particular, he stressed Moore's expertise in community policing and his dedication to "eradicat racism and brutality within the LAPD."
Personal life, death, and legacy
With his wife Harriett Johnson, Mack had three children. She died in 2016. Mack was 81 when he died of cancer on June 21, 2018, at Cedars-Sinai Medical Center in Los Angeles. For Los Angeles Mayor Eric Garcetti, Mack "made history with a fierce determination to pursue justice, an unshakable commitment to equality, and an unbreakable bond with the community he worked tirelessly to uplift every day of his remarkable life." The Los Angeles Times called him "one of the most powerful voices on Los Angeles police reform." John W. Mack Elementary School, part of the Los Angeles Unified School District since its 2005 inception, is named in his honor.