Born in Oakland, California, Armstrong received an Associate of Arts degree from Merritt College in 1967 and a Bachelor of Arts degree from California State University, Fresno in 1969.

[1] She then received a Juris Doctor from the University of San Francisco School of Law in 1977.

From 1979 to 1980, she was a senior consultant to the California Assembly Committee on Criminal Justice.

She was a trial attorney of Public Integrity Section of the United States Department of Justice from 1982 to 1983, and then served as a Commissioner on the Consumer Product Safety Commission from 1983 to 1986, and on the United States Parole Commission from 1986 to 1989.

[2] On April 25, 1991, Armstrong was nominated by President George H. W. Bush to a seat on the United States District Court for the Northern District of California vacated by William Austin Ingram.