Smith resigned from the Department of Justice on Friday according to court documents filed in federal court, Politico said.
A former war crimes prosecutor, Smith brought two of the four criminal cases Trump faced after leaving office, but saw them grind to a halt after a Trump-appointed judge in Florida dismissed one and the US Supreme Court -- with three justices appointed by Trump -- found that former presidents have sweeping immunity from prosecution for official acts. Neither case went to trial.
After Trump defeated Democratic Vice President Kamala Harris in the November 5 election, Smith dropped both cases, citing a longstanding Justice Department rule against prosecuting sitting presidents.
In asking courts to dismiss the charges, Smith's team defended the merits of the cases they had brought, signalling only that Trump's impending return to the White House made them untenable.
Smith's departure is another marker of the collapse of the criminal cases against Trump, which could end without any legal consequences for the incoming president.
Trump, who has frequently called Smith "deranged", had said he would fire him immediately upon taking office on January 20, and has suggested that he may pursue retribution against Smith and others who investigated him once he returns to office.
Trump in 2023 became the first sitting or former US president to face criminal prosecution, first in New York, where he was charged with trying to cover up a hush money payment to a porn star during his 2016 presidential campaign.
Smith's charges followed, accusing Trump of illegally retaining classified material after leaving office and of trying to overturn his 2020 loss, a campaign that sparked the January 6, 2021, assault on the US Capitol.
Prosecutors in Georgia also charged Trump over his efforts to overturn his election defeat in that state.
Trump denied wrongdoing and assailed the prosecutions as politically motivated attempts to damage his campaign. He raised millions in campaign contributions from courthouse appearances and used the cases to drive a powerful narrative that the political establishment was arrayed against him and his supporters.
The Justice Department defended the cases, saying they were run by career prosecutors who operated free of political influence.
The indictments, the first federal cases against a former US president, accused Trump of taking highly sensitive national security documents to his Florida resort and using false claims of voter fraud to attempt to derail the collection and certification of votes following his 2020 election loss.
"The attack on our nation's Capitol on Jan. 6, 2021, was an unprecedented assault on the seat of American democracy. As described in the indictment, it was fuelled by lies - lies by the defendant, targeted at obstructing the bedrock function of the US government," Smith said in announcing the election indictment in August 2023, one of only two public appearances he made during his investigation.
