The former army intelligence analyst was responsible for one of the largest leaks of classified documents in US history.
"After another anxious four months of waiting, the day has finally arrived. I am looking forward to so much!" Manning, whose sentence was commuted by former president Barack Obama, said in a statement via her legal team.
"Whatever is ahead of me, is far more important than the past," said the 29-year-old, who served seven of a 35-year sentence over one of the largest leaks of classified documents in US history.
"I'm figuring things out right now -- which is exciting, awkward, fun, and all new for me."
Manning, who twice attempted to commit suicide during her incarceration at Fort Leavenworth, was the unlikely source of disclosures that rocked the US government.
While serving as an army intelligence analyst in Baghdad in 2010, Manning -- then still a male soldier known as Bradley -- smuggled more than 700,000 classified documents to the WikiLeaks whisteblower organization.
The documents 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.
“It is incredible to witness Chelsea Manning's freedom after having seen and worked with her behind bars for four years," said Chase Strangio, a lawyer with the ACLU rights group that lobbied for Manning's early release.
"We can all finally truly celebrate the strength and heroism she has shown in surviving and sharing her truth and life with all of us," Strangio said.
"Through extended periods of solitary confinement and up against the government's insistence on denying her medical care and existence as a woman, Chelsea has emerged with grace, resilience, and an inspiring amount of love for others.”