A former son-in-law of President Donald Trump’s ex-campaign chairman pleaded guilty to federal criminal charges in a sealed proceeding in California in January and is cooperating with federal investigators, people familiar with the matter said.

Jeffrey Yohai, who until last year was married to one of Paul Manafort’s daughters, pleaded guilty to conspiracy charges in the Central District of California relating to real-estate loans on properties in New York and California, one of these people said. Mr. Yohai invested with Mr....