WASHINGTON — In his day job at the White House, Jofi Joseph was known as a generally collegial guy, a junior but experienced staff member working on some of the nation’s most serious business — specifically arms proliferation, and especially involving Iran.

But after hours? That is when he became @NatSecWonk: “I’m abrasive and I bring the snark,” he wrote anonymously on his now-closed Twitter account. “Unapologetically says what everyone else only thinks” on “the foreign policy and national security scene.”

Now that Mr. Joseph has been unmasked as @NatSecWonk, call him unemployed.

White House officials confirmed on Wednesday that Mr. Joseph, formerly the director of nuclear nonproliferation issues on the National Security Council staff, was fired last week after he was exposed as the author of numerous Twitter messages that over more than two years knocked public figures, reporters and even his colleagues. Mr. Joseph’s writings did not seem to divulge national security secrets, like details of new nuclear talks with Iran. They tended to be less substantive than sophomoric, skewering people in both parties.

“I can tell you that the individual does not work here anymore,” Jay Carney, the White House press secretary, told reporters.