Seymour was born in Battle Creek, Michigan in 1940, the second oldest of four children. Seymour and her family traveled extensively when she was young, visiting all but three states by car by the time she went to college. Though neither of her parents had a college education, they strongly influenced Seymour to obtain the highest level of education possible.[2] Seymour received a Bachelor of Arts degree from Smith College in 1962, graduating Phi Beta Kappa and magna cum laude, and her Juris Doctor from Harvard Law School in 1965.[3][4] At Harvard Law School, she was one of 23 women in a class of 550.[4]
Seymour was in private practice in Boston, Massachusetts from 1965 to 1966, in Tulsa, Oklahoma from 1966 to 1967, in Houston, Texas from 1968 to 1969, and in Tulsa again from 1971 to 1979. In Houston, she was the first woman hired by Baker Botts.[3]