Sabisky decided to resign even though Johnson's spokesman earlier had refused to condemn his views.
As well as the comments about black Americans, the aide, 27, also posted on Twitter that women's sport is "more comparable to the Paralympics than it is to men's," the Mail on Sunday reported, and in a 2016 interview with Schools Week magazine advocated eugenics - the practice of selective breeding.
Opposition politicians had called for Sabisky to be fired. He was appointed as part of a drive by Johnson's most powerful aide, Dominic Cummings, to hire mavericks, free thinkers and reformers to shake up the way the UK Government works from the inside.
Sabisky's exit marks another dramatic development in Johnson's Administration. Last week, Chancellor of the Exchequer Sajid Javid quit after Johnson made a grab to take full control over the Treasury.
It's not the first time Johnson's officials have had to deal with allegations of racism. Johnson himself has been criticised for articles he wrote for the Daily Telegraph newspaper, in which he used a racist term to refer to black people, and said Muslim women wearing the burqa looked like "letter boxes."
- Bloomberg