United States Navy Admiral Michael Mullen, then Chairman of the Joint Chiefs of Staff, said the leaks had placed the lives of American Soldiers and Afghan informants in danger. Journalist Glenn Greenwald argued that Manning was the most important whistleblower since Daniel Ellsberg leaked the Pentagon Papers in 1971. In an impromptu questioning session after a fundraiser, captured on a cell phone video, President Barack Obama said that Manning "broke the law", which was later criticized as "unlawful command influence" on Manning's upcoming trial.

Born Bradley Edward Manning in 1987, in Oklahoma City, Oklahoma, she was the second child of Susan Fox, originally from Wales, and Brian Manning, an American. Brian had joined the United States Navy in 1974, at the age of 19, and served for five years as an intelligence analyst. Brian met Susan in a local Woolworths store while stationed in Wales at RAF Brawdy. Manning's older sister, Casey Manning, was born in 1976. The couple returned to the United States in 1979, settling first in California. After their move near Crescent, Oklahoma, they bought a two-story house with an above-ground swimming pool and 5 acres (2 hectares) of land, where they kept pigs and chickens.

A childhood friend of Manning's, speaking about a conversation they had when Manning was 13, said: "he told me he was gay". The friend also said that Manning's home life was not good and that her father was very controlling. Around this time, Manning's parents divorced. She and her mother Susan moved out of the house to a rented apartment in Crescent, Oklahoma. Susan's instability continued, and in 1998 she attempted suicide; Manning's sister drove their mother to the hospital, with the 11-year-old Manning sitting in the back of the car trying to make sure their mother was still breathing.

Manning's father remarried in 2000, the same year as his divorce. His new wife, also named Susan, had a son from a previous relationship. Manning apparently reacted badly when the son changed his surname to Manning, too; she started taking running jumps at the walls, telling her mother: "I'm nobody now."

In November 2001, Manning and her mother left the United States and moved to Haverfordwest, Wales, where her mother had family. Manning attended the town's Tasker Milward secondary school. A school friend there told Ed Caesar for The Sunday Times that Manning's personality was "unique, extremely unique. Very quirky, very opinionated, very political, very clever, very articulate." Manning's interest in computers continued, and in 2003, she and a friend, James Kirkpatrick, set up an online message board, angeldyne.com, that offered games and music downloads.

After graduating from high school in 2005 at age 17, and fearing that her mother was becoming too ill to cope, Manning returned to the United States. She moved in with her father in Oklahoma City, where he was living with his second wife and her child. Manning got a job as a developer with a software company, Zoto, and was apparently happy for a time, but was let go after four months. Her boss told The Washington Post that on a few occasions Manning had "just locked up" and would simply sit and stare, and in the end communication became too difficult. The boss told the newspaper that "nobody's been taking care of this kid for a really long time".

WikiLeaks was set up in late 2006 as a disclosure portal, initially using the Wikipedia model, where volunteers would write up restricted or legally threatened material submitted by whistleblowers. It was Julian Assange—an Australian Internet Activist and Journalist, and the de facto editor-in-chief of WikiLeaks—who had the idea of creating what Ben Laurie called an "open-source, democratic intelligence agency". The open-editing aspect was soon abandoned, but the site remained open for anonymous submissions.

Manning began basic training at Fort Leonard Wood, Missouri, on October 2, 2007. She wrote that she soon realized she was neither physically nor mentally prepared for it. Six weeks after enlisting, she was sent to the discharge unit. She was allegedly being bullied, and in the opinion of another soldier, was having a breakdown. The soldier told The Guardian: "The kid was barely five foot ... He was a runt, so pick on him. He's crazy, pick on him. He's a faggot, pick on him. The guy took it from every side. He couldn't please anyone." Nicks writes that Manning, who was used to being bullied, fought back—if the drill sergeants screamed at her, she would scream at them—to the point where they started calling her "General Manning".

In an article following her recovery, entitled "Moving On", Chelsea reflected on her change in identity, wishing people to see her no longer as "Chelsea Manning, formerly Bradley Manning, a US Army Soldier... convicted..." but as a person. She used a selfie from 2008 to accompany the article.

During the Article 32 hearing, the prosecution, led by Captain Ashden Fein, presented 300,000 pages of documents in evidence, including chat logs and Classified material. The court heard from two Army investigators, Special Agent David Shaver, head of the digital forensics and research branch of the Army's Computer Crime Investigative Unit (CCIU); and Mark Johnson, a digital forensics contractor from ManTech International, who works for the CCIU. They testified that they had found 100,000 State Department cables on a workplace computer Manning had used between November 2009 and May 2010; 400,000 military reports from Iraq and 91,000 from Afghanistan on an SD card found in her basement room in her aunt's home in Potomac, Maryland; and 10,000 cables on her personal MacBook Pro and storage devices that they said had not been passed to WikiLeaks because a file was corrupted. They also recovered 14 to 15 pages of encrypted chats, in unallocated space on Manning's MacBook hard drive, between Manning and someone believed to be Julian Assange. Two of the chat handles, which used the Berlin Chaos Computer Club's domain (ccc.de), were associated with the names Julian Assange and Nathaniel Frank.

In an article written by Manning, she says her first public appearance as female was in February 2010 while on leave from her military duties; Manning was exhilarated to blend in as a woman.

In 2011, Manning was awarded a "Whistleblowerpreis" by the German Section of the International Association of Lawyers against Nuclear Arms and the Federation of German Scientists. In 2012, she was awarded "People's Choice Award" awarded by Global Exchange. In 2013, she was awarded the Sean MacBride Peace Prize by the International Peace Bureau. In 2014, she was awarded the Sam Adams Award by Sam Adams Associates for Integrity in Intelligence.

On August 22, 2013, the day after sentencing, Manning's attorney issued a press release to the Today show announcing that his client was a female, and asked that she be referred to by her new name of Chelsea and feminine pronouns. Manning's statement included the following:

After first getting in touch with Cassandra Fairbanks—an admirer and Writer for right-wing website The Gateway Pundit—in September 2017, Manning tapped into Fairbanks's close ties to D.C.-area alt-right media influencers. In December 2017, Manning participated with Fairbanks, Posobiec, Wintrich and others in Escape the Room DC, and spent an evening drinking and playing Cards Against Humanity at Wintrich's apartment with him, Fairbanks, and others. "I viewed this as an opportunity to use the Celebrity and fame I've gotten since getting out of prison," Manning told The Daily Beast in January 2018, "to gather information and to ultimately find ways in which we who are against the alt-right can undermine the alt-right." She added, "The thing in all this that I've learned is that they don't actually believe the things that they say. I just feel they're opportunists and that they exploit their Twitter followers' fears." Manning acknowledged, however, that these incidents left many of her own supporters feeling betrayed. "People have every right to be confused and hurt by this," she said. "Regardless of good intentions, I leveraged my privilege to gain access to spaces others couldn't dream of entering safely. I never meant to hurt my supporters. No amount of information on the alt-right is worth losing the trust of my supporters."

In April 2015, Manning began communicating via Twitter, under the handle @xychelsea, by using a voice phone to dictate to intermediaries, who tweeted on her behalf.

On September 9, 2016, Manning began a hunger strike to protest what she described as her being bullied by prison authorities and the U.S. government. On September 13, the ACLU announced that Manning had ended the five-day hunger strike after the Army agreed to provide gender transition surgery. The operation, however, was not performed before her release from prison in May 2017.

During an October 8, 2017 appearance at The New Yorker Festival, Manning said she is legally unable to speak about certain details concerning her leaks, confirming a July 2017 post from her verified Twitter account saying "technically, i cant [sic] read, comment on, discuss, or even look at any leaked material, even if it was after 2010".

On January 20, 2018, Manning attended "A Night for Freedom" hosted by far-right social media personality Mike Cernovich at the nightclub FREQ in Hell's Kitchen, Manhattan. The party was billed, in Cernovich's words, as a "gathering of patriots and political dissidents who are bored with mainstream political events," and included right-wing figures such as Gavin McInnes, James O'Keefe, Lucian Wintrich, and Jack Posobiec. According to The Washington Post, Manning's attendance infuriated the far left. "What followed," The Post reported, "was an overheated Internet tug-of-war between opposite sides of the political spectrum, each accusing the other of co-opting Manning, while her intentions were relentlessly picked apart." Manning afterwards stated that she was acting as a double agent, infiltrating the alt-right to gather information and insight about alt-right rally plans.