Florence Jessie MacWilliams (1917–27 May 1990) was an English mathematician who contributed to the field of coding theory. She was born in Stoke-on-Trent, England and studied at the University of Cambridge, receiving her BA in 1938 and her MA in the following year. She moved to the United States in 1939 and studied at Johns Hopkins University. One year later she left Johns Hopkins for Harvard University. In 1955 she became a programmer and learned coding theory at Bell Labs where she spent most of her career. Although she did major research at Bell Labs, she was denied a promotion to a mathematics research position until she received a Ph.D. She would proceed to fulfill some of the PhD's requirements while working at Bell Labs and taking care of her family, but she completed her PhD after returning to Harvard for one more year (1961-1962). She and her daughter Anne were both studying mathematics at Harvard that year. She worked on error-correcting codes and co-wrote The Theory of Error-Correcting Codes with Neil Sloane. She is known for the MacWilliams identities in coding theory.