https://www.supremecourt.gov/about/faq_general.aspx
"The Constitution does not specify qualifications for Justices such as age, education, profession, or native-born citizenship. A Justice does not have to be a lawyer or a law school graduate, but all Justices have been trained in the law. Many of the 18th and 19th century Justices studied law under a mentor because there were few law schools in the country.
The last Justice to be appointed who did not attend any law school was James F. Byrnes (1941-1942). He did not graduate from high school and taught himself law, passing the bar at the age of 23.
Robert H. Jackson (1941-1954). While Jackson did not attend an undergraduate college, he did study law at Albany Law School in New York. At the time of his graduation, Jackson was only twenty years old and one of the requirements for a law degree was that students must be twenty-one years old. Thus rather than a law degree, Jackson was awarded with a "diploma of graduation." Twenty-nine years later, Albany Law School belatedly presented Jackson with a law degree noting his original graduating class of 1912."