Alice Johnson, a woman granted clemency by United States President Donald Trump, has thanked her "war angel" Kim Kardashian West for lobbying for her release and meeting with Trump in a bid to see her freed.

63-year-old Johnson was jailed in 1996 for a non-violent drug charge, with Kardashian West joining a long-running campaign for her release and meeting with Trump to discuss prison reform, having become aware of Johnson's case via social media.

On her release, Johnson thanked Kardashian West for getting involved in her case, describing her as her "war angel" and thanking her for her dedication to Johnson's freedom.

"This is a miracle day, this is one of the greatest days of my life, the most unexpected thing to happen to me in my life... When they all came on, then I heard Kim Kardashian's voice and she was the one that told me it had happened, that I was free, that I was going to rejoin my family," Johnson told reporters outside the prison in Pickens County, Alabama.

"Kim thank you. I've always called her my angel, but with this she turned into my war angel, only war angels never give up, she has  truly been relentless in her fight for me, and to know a woman who has never met me, who has embraced my story and taken me into her heart...

"And this is not just a publicity thing for her, Kim told me that when she left that White House meeting, that no matter how this turns out, that she would never stop fighting for me until I got home. She is an incredible woman, an amazing woman."

Kardashian West tweeted that calling Johnson to tell her that she was free is a moment she will never forget.

On Johnson's release the White House released a statement attributing her pardon to her being a "model prisoner."

"Ms Johnson has accepted responsibility for her past behaviour and has been a model prisoner over the past two decades," the statement reads.

"Despite receiving a life sentence, Alice worked hard to rehabilitate herself in prison, and act as a mentor to her fellow inmates."