The US soldier then known as Pte First Class Bradley Manning lip-synced to Lady Gaga while downloading thousands of classified documents from military servers, according to a computer hacker she befriended.
Now, the 25-year-old soldier has been sentenced to 35 years in prison after being convicted of 20 charges in connection with the leaks, including espionage. She was acquitted of the most serious charge, aiding the enemy.
After her sentencing, Pte Manning, who was born a man, said she wanted to live as a woman and had taken the name Chelsea.
As an intelligence analyst in the US Army, Pte Manning was given access to a large amount of highly sensitive information.
But as a private first class, she was very low-ranking with a relatively meagre wage.
'Funny little character'
According to her friends, she had become frustrated with a military career that appeared to be stagnating.
And her personal life appears to have hit a downward spiral after she was posted to Iraq in 2009.
Pte Manning joined the army in 2007 after drifting through low-paid jobs.
She had been brought up in Crescent, a small town in Oklahoma. Her father, Brian, had reportedly spent five years in the military.
But her parents divorced when she was a teenager, and she moved with her mother to Haverfordwest in south-west Wales.
As a teenager, she was said to have been a hothead who was often teased for being a geek.
"He would get upset, slam books on the desk if people wouldn't listen to him or understand his point of view," a classmate from Oklahoma, Chera Moore, told the New York Times.
A friend from Wales, James Kirkpatrick, told the BBC she was a "funny little character, really on the ball" who was obsessed with computers.
After finishing school, she returned to the US and joined the Army. Friends say she enlisted to help pay for college, and she later said she had joined up in the hope the Army would rid her of her desire to be a woman.
She was deployed to Iraq in October 2009. But messages she posted on Facebook suggest she was far from happy.
She wrote in early May 2010 on the site that she was "beyond frustrated with people and society at large".
Other status updates followed, among them: "Bradley Manning is not a piece of equipment."
A week earlier, she had written: "Bradley Manning is now left with the sinking feeling that he doesn't have anything left."
Some of the postings appear to refer to a recent breakdown of a relationship.
But weeks later her words appeared prophetic when she was arrested by military investigators on suspicion of stealing secret information.
Self-confessed computer hacker Adrian Lamo told the world's media how Pte Manning had admitted the data theft during conversations they had on the internet.
"Listened and lip-synced to Lady Gaga's Telephone while exfiltrating possibly the largest data spillage in American history," Pte Manning wrote, according to a transcript of their messages published on Wired's website.
"Weak servers, weak logging, weak physical security, weak counter-intelligence, inattentive signal analysis… a perfect storm."
Mr Lamo went to the authorities with the messages.
During a sentencing hearing at her court martial in August 2013, a military psychiatrist testified that Pte Manning had struggled with her gender identity and wanted to become a woman.
She also said Pte Manning had felt abandoned by friends and family after her 2009 deployment to Iraq and that a relationship with a boyfriend had hit a rough patch.
An apt location
In March 2011, the US Army charged Pte Manning with 22 counts relating to the unauthorised possession and distribution of more than 720,000 secret diplomatic and military documents.
Among the files she passed to Wikileaks was video footage of an Apache helicopter killing 12 civilians in Baghdad in 2007.
Wikileaks released tens of thousands of documents relating to the Afghan war.
The website later disclosed thousands of sensitive messages written by US diplomats and military records from the Iraq war, causing growing embarrassment to the US government.
Pte Manning told the court she had leaked the documents to spark a public debate in the US about the role of the military and about US foreign policy.
At a later sentencing hearing, she apologised for "hurting the US" and said she had mistakenly believed she could "change the world for the better".
And she said that in retrospect, she should have worked "inside the system".
Officials say the location of the trial, Fort Meade, was chosen because its military court is one of the largest in the Washington area.
A day after she was sentenced to 35 years in prison, Pte Manning said she wanted to live as a woman.
"I am Chelsea Manning," she said in statement to NBC's Today programme. "I am a female."
She said she had felt female since childhood, wanted at once to begin hormone therapy, and wished to be addressed as Chelsea.
When asked why Pte Manning was making this announcement after her sentencing, her lawyer, David Coombs said: "Chelsea didn't want to have this be something that overshadowed the case."