In a statement, Google said an investigation found Margaret Mitchell had moved files outside the company.
The ethics unit has been under scrutiny since December, following the departure of another senior figure, Timnit Gebru.
Both women had campaigned for more diversity at Google and raised concerns about censorship within the company.
Dr Mitchell announced the news in a tweet, which read "I'm fired".
For the past five weeks she had been locked out of Google's systems, including her emails and calendars.