The two have had a strained relationship ever since Sessions recused himself from the Russian Federation probe - Trump did not wait long for the midterm dust to settle to seek his resignation, which immediately put congressional Democrats on high alert for the possibility of any interference with Special Counsel Robert Mueller's investigation. He also tweeted a link describing the probe as a "witch hunt".
The department confirmed he would be assuming oversight of the Mueller probe from Deputy Attorney General Rod Rosenstein.
Whitaker, a former US attorney for the Southern District of Iowa, had served as Sessions's chief of staff since September 2017. "This is a break-the-glass moment", he said.
"I'm disappointed in the attorney general for many reasons", Trump told reporters in September. The next attorney general could enjoy some more flexibility when it comes to enforcing federal marijuana laws.
High-ranking Democrats reacted with alarm, calling for Mr Whitaker to recuse himself from the investigation.
Christie is a longtime friend of President Trump's and endorsed him after dropping out of the 2016 presidential campaign.
And I think that's what's critical.
That left Rosenstein overseeing the investigation, and he named Mueller as special counsel. He's won guilty pleas and agreements to cooperate from Trump's campaign chairman Paul Manafort and his former National Security Adviser Michael Flynn.
"We are immediately issuing multiple letters to key officials demanding that they preserve all relevant documents related to this action to make sure that the investigation and any evidence remains safe from improper interference or destruction", Nadler said. He can also block Mr Mueller from subpoenaing Mr Trump or bringing an indictment, although he must notify Congress if he does so.
The departure was the culmination of a toxic relationship that frayed just weeks into Sessions' tenure, when he stepped aside from the Russia investigation because of his campaign advocacy and following the revelation that he had met twice in 2016 with the Russian ambassador to the U.S. Today, Graham did not seem to indicate any "holy hell" was on the way, saying in a statement: "I look forward to working with President Trump to find a confirmable, worthy successor so that we can start a new chapter at the Department of Justice and deal with both the opportunities and challenges our nation faces".
The switch, announced on Trump's Twitter feed, provoked instant consternation across Washington, where politicians from both sides of the aisle have long warned that political interference in Mueller's work can not be tolerated.