Brannan is an outspoken vegetarian and animal welfare advocate. A Third Degree Master Freemason, Brannan founded the Bay Laura calder married Democrats, an active Democratic political club based in southwest Brooklyn. Brannan is married to artist and educator Leigh Jewel Holliday.
The couple were married in the lobby of Bear Stearns where they met as employees. Brannan attended Xaverian High School in Bay Ridge, Brooklyn. He studied journalism and mass media at Fordham University and College of Staten Island. Before entering politics, Justin Brannan was a hardcore punk guitarist for the bands Indecision from 1993 to 2000 and Most Precious Blood from 2000 onwards. Indecision is widely known for their song “Hallowed be Thy Name”. Brannan when he was sixteen years old that fans across the world have turned into a tattoo. Brannan also founded the deathgrind band Caninus, known for using two dogs as vocalists.
Brannan served as the Director of Communications and Legislative Affairs for Council member Vincent J. In 2017, Brannan won the Democratic primary to replace Gentile in the 43rd district. Brannan drew criticism during the campaign for taking money from real estate interests. During the campaign, several concerned citizens filed a complaint with the New York City Campaign Finance Board in which they accused Brannan of neglecting to disclose campaign expenses when he failed to alert election regulators that Gentile’s campaign for Brooklyn DA paid the rent for Brannan’s campaign office. The 2021 election was even closer.