With little fanfare, Manning, 29, walked out of Fort Leavenworth's military jail at 2 am to start a new life, after former president Barack Obama commuted her 35-year sentence just days before leaving office.
"After another anxious four months of waiting, the day has finally arrived. I am looking forward to so much!" said Manning in a statement released by her legal team.
She posted a picture on Twitter of her black-and-white Converse baseball shoes with the caption "First steps of freedom!!" followed by a smiley-face emoticon.
"Whatever is ahead of me, is far more important than the past," said Manning, whose original release date was set for 2045. "I'm figuring things out right now -- which is exciting, awkward, fun, and all new for me."
Manning twice attempted suicide in jail, the second time in an isolation cell where she had been sent as punishment for the first attempt.
Manning was arrested in July 2010 over the release of a huge trove of more than 700,000 classified military and diplomatic documents via WikiLeaks.
Lauded as a hero by freedom of speech advocates and as a traitor by others, including President Donald Trump, Manning is expected to head to Maryland, where she lived with an aunt before joining the army as a young man.
The former military intelligence analyst struggled with gender identity issues while in the service, and eventually began her transition while locked up in the all-male prison in Kansas.
"It's going to be a profound change for her," Evan Greer, a friend of Manning, told AFP.
"Understandably she has a lot of question marks about what her life is going to be like next, but it's also her first chance in her adult life that she is really going to be able to define that for herself," Greer said, noting that Manning joined the army at a very young age.
"She is a very driven person and I know she intends to continue what she has always done, which is advocating for others and using her voice to try to make the world a better place so I'd expect that we will be hearing from her before too long," said Greer. She said Manning's exact location would be kept under wraps for her own security and privacy.
Maintaining that privacy could prove difficult however. Now an internationally known figure, both adored and reviled, Manning is even the subject of a planned documentary called "XY Chelsea" to be co-produced by Oscar winner Laura Poitras, who worked with fellow intelligence leaker Edward Snowden.
"Chelsea Manning's story is one of the major events of our time," the film-makers said as they announced the project at the opening of the Cannes film festival, the same day that Manning was released.
The documents leaked by Manning ranged from embarrassing diplomatic cables that revealed how US envoys really felt about friends and foes alike, to videos showing a US air strike in Afghanistan in 2009 that left more than 100 civilians dead and footage of a US helicopter attack in Iraq that killed two Reuters journalists.
Manning supporters said they feared she would not have survived the long jail sentence. Aside from suicide attempts, she had also been put in solitary for a hunger strike in protest at her treatment as a transgender woman in a men's prison.
"I firmly believe that the fact she is walking out of prison today not only secured her freedom but also very likely saved her life," said Greer.
Manning, a native of Oklahoma, had a difficult childhood, seeing her parents divorce and being mocked as effeminate as a young boy named Bradley.