What president became a U.S. Senator after he left office?
That distinction goes to the country's 17th president, Andrew Johnson.
Johnson, who was a former representative, governor and senator from Tennessee, was successfully impeached in 1869 for attempting to remove his disloyal Secretary of War without approval from the U.S. Senate.
He returned to Tennessee and ran for the U.S. Senate again in 1870 and 1872, failing both times. However, in 1874 he became the first and only president to win a Senate seat after serving as President of the United States. He took his seat on March 5, 1875.
Unfortunately, Johnson suffered a stroke and died four months later.
Here is Johnson's obituary from the New York Times.