Born in Syracuse, New York, Gillmor received a Bachelor of Arts degree from Queens College, City University of New York in 1965, and a Bachelor of Laws from Boston University School of Law in 1968.

She was a law clerk to William S. Richardson, Chief Justice of the Hawaii State Supreme Court in 1972, a deputy public defender of the Honolulu Office of the Public Defender from 1972 to 1974, and a lecturer at the University of Hawaii in 1975.

In November 2012, Gillmor found that Hawaii's licensing requirement to openly carry firearms did not violate the Second Amendment to the United States Constitution.

[2] Her judgment was then reversed by a divided panel of the United States Court of Appeals for the Ninth Circuit in July 2018.

[3] The ruling was re-heard en banc by the 9th Circuit and Judge Gillmor's decision was affirmed in March 2021.