Who was the first president to serve on the surpreme court?

None. William Howard Taft served both as President and Chief Justice of the US Supreme Court, but he was President first, from 1909-1913. President Warren G. Harding later nominated Taft as Chief Justice of the United States (Supreme Court), where he served from 1921-1930.

Charles Evans Hughes resigned from the Supreme Court to run for President in 1916, but he was not Chief Justice and he was not elected President. He later returned to the supreme court as the Chief Justice.

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