Choi, the daughter of a shadowy religious figure, had been at the Park's side for decades, bound to the politician by her father.
Tuesday's verdict comes just days after an appeals court freed Samsung scion Lee Jae-yong after a year in jail.
According to the judge who presided over the trial, Choi accepted around 14 billion won from Samsung and Lotte and that she also "meddled in the state affairs widely". Former presidential aide An Chong-bum was sentenced to six years of prison and a fine of 100 million won for corruption.
The massive scandal involving Choi and Park triggered weeks of street protests calling for Park to step down.
Yet the supposed mastermind behind presidential policymaking was hardly a sophisticated operator. He subsequently turned against her, and released incriminating evidence.
A lawyer for Choi said she will appeal the sentence. "This is a heavy sentence bordering on cruelty", he told local reporters.
The scandal brought down Park and has implicated business leaders from Samsung and other corporations.
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One large law firm declined to comment on the sentencing, but a foreign legal consultant working in Korea suggested that the judges in the case were reactive to public feeling, or what has been dubbed, "the law of public opinion".
"Twenty years for a white-collar crime is exceptional and it is hard to see the sentence being reduced on appeal, because the court is so obviously conscious of public outrage that someone had to be held responsible", said Brendon Carr, a senior foreign legal consultant at Seoul law firm Hwang, Hong and Co.
That responsible party is clearly Choi - who extracted the bribes - rather than the parties who offered them.
The prison term for Shin follows a December ruling in which the court found him guilty of breach of trust and embezzlement in a different case but suspended sentencing, leaving him free to run the group.
Prosecutors originally demanded 25 years in prison for Choi, with a fine of 118.5 billion won (109 million US dollars) and a forfeit of 7.79 billion won (7.18 million USA dollars).
Choi pleaded not guilty, claiming she never pursued any personal benefits but attempted to help Parks work, lashing out at the prosecution for "framing" her.
The prosecutors had demanded 25 years in prison, a fine of 118.5 billion won and forfeiture of 7.7 billion won for Choi, arguing the civilian friend took advantage of her 40-year ties to Park and abused authority to disrupt the order of the government and private businesses.
Park is also on trial on charges of corruption, coercion and leaking confidential information, which she has denied.