Mook joined Hillary Clinton's 2008 presidential campaign in 2007. He served as the campaign's state director for Nevada, Indiana, and Ohio. Clinton won the popular vote in all three states. Mook then managed Jeanne Shaheen's successful campaign for the United States Senate that fall. Mook joined the Democratic Congressional Campaign Committee in 2009 as their political director, and was named independent expenditure director of the DCCC in May 2010. After the 2010 House of Representatives elections, where the Democrats lost the majority, Mook was named executive director. In the 2012 House of Representatives elections, he aided the Democrats in gaining eight seats, though Democrats had aimed for the 25 seats needed to retake the majority.
In 2013, Mook left the DCCC and was named the campaign manager of Terry McAuliffe's gubernatorial campaign. That year, Politico named Mook one of their "50 Politicos to Watch." Mook led McAuliffe's campaign to victory. He worked for McAuliffe's political action committee as well as the Virginia Progress PAC, helping in the reelection campaign of Senator Mark Warner in 2014.
In January 2015, Clinton hired Mook and Joel Benenson as strategists. Upon the April 2015 announcement of Clinton's 2016 campaign for president, Mook was introduced as Clinton's campaign manager. According to the Washington Post, as Clinton's campaign manager Mook won praise "both inside the campaign and among Clinton's vast circle of second-guessers, for the airtight and drama-free campaign he has built." A group of about 150 young political operatives close to Mook became known as the "Mook Mafia." Mook played a key role in negotiating with Bernie Sanders' campaign to win his endorsement for Clinton. During the campaign, Donna Brazile commented on Mook's micro-tagging of voters based on purchasing preferences, alleging that it "missed the big picture."
Aftermath
In the aftermath of the campaign, former staffers and political pundits criticized Mook for actions taken during the campaign. In particular, they faulted what they considered his excessive focus on data analytics. In 2017, in response to Russia's interference in the 2016 presidential election, Mook, together with Matt Rhoades, started the "Defending Digital Democracy" initiative at the Belfer Center for Science and International Affairs at the Harvard Kennedy School. In January 2017, Leading Authorities, a Washington, D.C.-based speaker's bureau, falsely announced that Mook had teamed with Trump campaign manager Corey Lewandowski to offer insights into the 2016 race. Mook and Lewandowski questioned the announcement, and Mook fired the firm as a result.
Personal life
Mook was the first openly gay manager of a major presidential campaign. In September 2017, Mook was announced as Harvard Institute of Politics' 2017–2018 Visiting Fellow.