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.
Dr Mitchell was a fierce critic of Google, and had expressed concern about Dr Gebru's departure from the company.
Dr Gebru, a leading Artificial Intelligence ethics researcher, says she was fired late last year after sending an internal email that accused Google of "silencing marginalised voices". However, Google claims she left the company.
She co-authored a research paper which she says she was asked to retract. The paper had pinpointed flaws in AI language technology, including a system built by Google.