There is no law mandating U.S. Supreme Court Justices be a United States citizen or even a resident of the United States. However, historically, the majority of U.S. Supreme Court Justices were or are citizens born in the United States and all of them resided in the United States when they were appointed.
Article I of the U.S. Constitution established citizenship requirements for the House of Representatives and the Senate. Representatives and Senators must reside in the states from which they're elected and be a United States citizen for seven and nine years, respectively. Article II of the U.S. Constitution restricted the Presidency to natural born U.S. citizens (or U.S. citizens at the time of the Constitution's adoption) and at least fourteen-year-residents of the United States. Article III established the Supreme Court of the United States and does not include any citizenship or residency requirements for justices.
The U.S. Constitution at Section 8 of Article I gave Congress authority to pass laws needed to execute its powers, one of which is to create and administer courts beneath the U.S. Supreme Court. Congress passed "The Judiciary Act of 1789" to initially establish the U.S. court system. In this act, Congress did not delineate any citizenship or residency requirements for Supreme Court Justices. Though Congress has passed laws to supplement or revise this original act over the years, it still has not specified any citizenship requirement for Supreme Court Justices.
There has been no statutory guidance regarding citizenship and residency for Supreme Court Justices. However, according to the U.S. Supreme Court's website, of the over one hundred individuals to have served as Supreme Court Justices, only six were born in a country other than the United States; the term of service for one ended in the 1700s, one in the 1800s and three in the 1900s. The most recent was Felix Frankfurter who was born in Vienna, Austria and served from 1939 to 1962. Additionally, every justice was appointed from one of the United States.
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