Kidd was born Mildred Virginia Kidd in the Germantown district of Philadelphia, Pennsylvania. She was the youngest daughter of Charles Kidd, a printer, and Zetta Daisy Whorley. She had polio at age 2. She was paralyzed for a year from the chest down. Growing up she attended the Berlitz School of Languages where she developed fluency in Spanish, Latin, Italian, French, and German. Kidd discovered science fiction at age 9. She became an active science fiction fan. She was a Futurian and, in 1941, became one of the founding members of the Vanguard Amateur Press Association. She did not attend college, saying "because I couldn't go to the University of Chicago, and I wouldn't go to any other." She married opera singer Jack Emden in 1943. They divorced in 1947. She then married the writer James Blish. They were married until 1963. She had four children: Karen Anne Emden (born 1944), Asa Benjamin Blish (born and died 1947), Dorothea Elisabeth Blish (born 1954), and Charles Benjamin Blish (born 1956).