A writer for the Fox News star Tucker Carlson resigned after he was revealed as the pseudonymous author of several years’ worth of racist, sexist and obscene posts on an online message board.

Blake Neff, who joined “Tucker Carlson Tonight” in 2017, left Fox News on Friday after the network learned of his activity on an online forum, AutoAdmit, which is popular with law students. The site has previously been the subject of at least one lawsuit for its offensive and misogynist content.

In a memo on Saturday, leaders of Fox News described Mr. Neff’s writings as “horrendous and deeply offensive” and said the network condemned his behavior.

“Neff’s abhorrent conduct on this forum was never divulged to the show or the network until Friday, at which point we swiftly accepted his resignation,” the network’s chief executive, Suzanne Scott, and its president and executive editor, Jay Wallace, wrote in the memo, which was distributed to the Fox News staff. “Make no mistake, actions such as his cannot and will not be tolerated at any time in any part of our work force.”

Mr. Carlson has not commented on the matter since Mr. Neff resigned. Ms. Scott and Mr. Wallace wrote in their memo that Mr. Carlson would address the episode on his Monday show.

Mr. Neff did not respond to inquiries for comment.

A conservative writer who previously worked at the right-wing news and opinion site The Daily Caller, which Mr. Carlson co-founded, Mr. Neff published on AutoAdmit under a pseudonym, CharlesXII.

His posts there mocked and denigrated African-Americans, Asian-Americans and women, and he contributed to message threads in which other writers used racial slurs. He also occasionally bragged about his influence on Mr. Carlson’s show.

CNN identified Mr. Neff as the author of the posts and first reported his resignation on Friday.

Mr. Neff, in a recent interview with the Dartmouth Alumni Magazine, said that when Mr. Carlson read off his teleprompter, “the first draft was written by me.” He told the magazine that he and Mr. Carlson agreed on many issues and that he enjoyed working at a show that could affect national politics.

“We’re very aware that we do have that power to sway the conversation, so we try to use it responsibly,” Mr. Neff told the magazine, which identified him as a 2013 graduate and a former editor at The Dartmouth Review, the college’s undergraduate conservative newspaper. (As of Saturday, the Dartmouth Alumni Magazine appeared to have removed the article about Mr. Neff from its website.)

Mr. Carlson is the No. 1-rated star of Fox News, and his program’s viewership has soared in recent months as he has taken a hard-line stance against national demonstrations over police brutality and racial injustice. Mr. Carlson has dismissed demonstrators as “criminal mobs” and warned that the rise of the Black Lives Matter movement could lead to anarchy and violence.

His rhetoric has prompted a backlash from major advertisers, including T-Mobile and the Walt Disney Company, many of which have instructed Fox News to prevent their spots from airing during Mr. Carlson’s show.

In recent days, Mr. Carlson faced criticism for on-air attacks against Senator Tammy Duckworth of Illinois, a Thai-American Democrat and a veteran who lost her legs fighting in Iraq. His broadside came after Ms. Duckworth, a potential vice-presidential nominee, said that she was open to arguments for removing statues of George Washington because he owned slaves.

Mr. Carlson called Ms. Duckworth a “vandal” and a “moron” and questioned her patriotism. Ms. Duckworth responded by writing that Mr. Carlson “doesn’t know what patriotism is.”

Mr. Neff was featured in a Washington Post dating column in 2017, in which he was quoted saying his hobbies “allow me to escape women.” He also told The Post he was miffed at his date’s assertion that she was not sure about starting a relationship, saying, “If I were Brad Pitt, you would be.”