The termination marks a remarkable downfall for Strzok, a 22-year veteran of the bureau who investigated Russian spies, defence officials accused of selling secrets to China and myriad other important cases.
In the twilight of his career, Strzok was integral to two of the bureau's most high-profile investigations: the Russia case; and the investigation into Hillary Clinton's use of a private email server while she was secretary of state.
But when a Justice Department inspector general investigation uncovered politically charged messages that Strzok had exchanged with another FBI official, he was relegated to a position in human resources.
Conservatives soon made Strzok the face of their attacks against the Special Counsel investigation into the President's campaign, and the FBI took steps to remove Strzok from its ranks.
President Donald Trump today used Strzok's firing to suggest the Russia investigation should be dropped, and the Clinton case redone.
"Agent Peter Strzok was just fired from the FBI - finally. The list of bad players in the FBI & DOJ gets longer & longer. Based on the fact that Strzok was in charge of the Witch Hunt, will it be dropped? It is a total Hoax. No Collusion, No Obstruction - I just fight back!" he wrote.
Minutes later, he added, "Just fired Agent Strzok, formerly of the FBI, was in charge of the Crooked Hillary Clinton sham investigation. It was a total fraud on the American public and should be properly redone!"
Strzok's team launched a GoFundMe page with a lengthy statement to raise money for his "legal costs and lost income," and said on the site that his firing was "apparently driven by political pressure."
Because Strzok is a senior-level FBI employee, and because the FBI's No. 2 official directed his firing, he has few realistic avenues left to get back his job. It's unclear if he plans to pursue legal action against the bureau.
Strzok's position in the bureau had been precarious since last summer, when the Department of Justice's inspector general, Michael Horowitz, told Special Counsel Robert Mueller that the lead agent on his team had been exchanging anti-Trump messages with an FBI lawyer. The next day, Mueller expelled Strzok from the group.
The lawyer, Lisa Page, had also been a part of Mueller's team, though she left a few weeks earlier and no longer works for the FBI. She and Strzok were having an affair.
Trump has previously derided the pair as "FBI lovers," and he and his conservative allies have pointed to their conduct in an attempt to discredit the Mueller probe. Before the firing was known publicly, Trump tweeted an attack on Strzok, Page and former FBI director James Comey and deputy director Andrew McCabe.
"Will the FBI ever recover it's once stellar reputation, so badly damaged by Comey, McCabe, Peter S and his lover, the lovely Lisa Page, and other top officials now dismissed or fired?" Trump wrote on Twitter. "So many of the great men and women of the FBI have been hurt by these clowns and losers!"
Horowitz concluded that Strzok showed a "willingness to take official action" to hurt Trump's electoral prospects, particularly in a text he sent telling Page "we'll stop" Trump from being president.
Strzok, who was a deputy assistant director for counterintelligence at the bureau, has apologised for sending the messages and said they reflected personal views that did not affect his work.
His lawyer has said that had Strzok wanted to prevent Trump's election, he could have leaked that Trump's campaign was under investigation for possibly coordinating with Russia - a revelation that might have upended his bid to become president.
