[6] Jesse Mandela Barnes was born in Milwaukee on December 1, 1986,[2][7][8] the son of a public school teacher and a United Auto Workers member.

[10] In August 2019, Barnes admitted that he had never officially completed his bachelor's degree in 2008 due to incomplete coursework in one class that he called a "minor technical issue", contrary to previous statements that he had graduated from Alabama A&M.

[14] Barnes worked for various political campaigns and in the office of Milwaukee Mayor Tom Barrett, eventually becoming an organizer for M.I.C.A.H., a Milwaukee-based interfaith coalition that advocates social justice.

[17] In April 2012, Barnes announced his candidacy for Wisconsin's 11th Assembly district, representing the north side of Milwaukee and a small part of Wauwatosa, challenging incumbent Jason Fields in the Democratic primary election.

It would have barred judges from considering the "nature, number and gravity" of the charges and required the release of a defendant unless there was "clear and convincing evidence" of flight risk or of danger to an individual or witness.

He also chaired the legislature's Black and Latino Caucus and helped lead a number of international delegations to the Middle East and southeast Asia.

[27] On April 11, 2016, Barnes announced that he would resign from the Assembly to launch a primary challenge against Lena Taylor, the Democratic incumbent in Wisconsin's 4th State Senate district.

[28] Lee framed the race as reflecting the broader struggle in the Democratic Party, pitting a young progressive challenger against an older, more centrist incumbent.

Evers and Barnes won the November 2018 election, narrowly defeating incumbent Republicans Scott Walker and Rebecca Kleefisch, 49.5% to 48.4%.

[44] On August 24, 2020, the day after the shooting of Jacob Blake, in Kenosha, Wisconsin, Barnes said it was "not an accident", and "This wasn't bad police work.

[52] In February 2023, Barnes launched The Long Run PAC to help the U.S. political campaigns of "pro-democracy" Democratic candidates who are young, persons of color, members of the LGBTQ community, and/or have working class backgrounds.

"[61] He supported red-flag laws, universal background checks for gun sales, and bans on privately made firearms, assault weapons, and high-capacity magazines.

"[66][67] Barnes supports a Green New Deal,[62][51] a $15 per hour minimum wage,[49] eliminating cash bail nationwide,[26] and legalizing marijuana.