A Delaware judge has tossed out a disinformation expert’s defamation lawsuit against Fox News, concluding that she either had failed to show that the statements at issue were false or that they were matters of opinion.

Nina Jankowicz was the former executive director of a Department of Homeland Security division tasked with monitoring the threat of disinformation. Last year, she sued the network and parent Fox Corp. in Delaware Superior Court, claiming that the network conducted a “malicious campaign of destruction” against her. The case was later moved to federal court.

Her lawsuit alleged that the network falsely claimed that she was trying to censor Americans’ speech, that she was fired from her position at the federal government, and that she wanted to give verified Twitter users, including herself, the power to edit others’ tweets.

But Colm F. Connolly, chief judge of U.S. District Court in Delaware, tossed...