[2] His father was a physics professor at Dartmouth College in Hanover, and his mother was a hospital administrator at Dartmouth–Hitchcock Medical Center, in nearby Lebanon, New Hampshire.

[3][4] Mook attended Hanover High School, where Matt Dunne, a member of the Vermont House of Representatives, served as the theater director.

[4][5][6] At Columbia, he starred in the 108th Varsity Show, along with Michelle Collins, Brandon Victor Dixon, Lang Fisher, Peter Koechley, Will Graham, Gabe Liedman, and Jenny Slate.

He joined the Democratic National Committee after Dean lost the nomination to John Kerry, serving as director of the get out the vote effort in Wisconsin during the general election.

[12] 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.

[19] During the campaign, Donna Brazile commented on Mook's micro-tagging of voters based on purchasing preferences, alleging that it "missed the big picture".

[1][21] 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.

[22] 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.

[24] During testimony given in May 2022 in the trial of Michael Sussmann, Mook testified that Clinton agreed to give a reporter information that connected Trump with Russian Alfa-Bank, even though her campaign was uncertain about whether the allegations were true.