Born in Oakland, California, Saundra Brown Armstrong received an A.A. from Merritt College in 1967 and a B.A. from California State University, Fresno in 1969. She was a police officer in the Oakland Police Department from 1970 to 1977. She then received a J.D. from the University of San Francisco School of Law in 1977. She was a Judicial extern, California Court of Appeals in 1977, and was a deputy district attorney in Alameda County, California from 1978 to 1979 and from 1980 to 1982. From 1979 to 1980, she was a senior consultant to the California Assembly Committee on Criminal Justice.
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 A. Ingram. She was confirmed by the United States Senate on June 14, 1991, and received her commission on June 18, 1991. She assumed senior status on March 23, 2012.