Denver's trial was scheduled for Sept. 3, but he and his attorney showed up last week at the Mercer County Magistrate Court office in Princeton, W.Va.
Denver declined comment Thursday.
"His attorney just brought him in right out of the blue," said a secretary who asked that her name not be used. "He wasn't on the docket for that day. I didn't know he was here until I looked up and saw him."
The original charge of possession of more than 15 grams of marijuana was reduced to simple possession of marijuana, court records showed.
Denver was arrested June 4 after police were notified that a package containing marijuana was sent by a delivery service to Denver's Princeton home.
After an officer delivered the package, a drug task force executed a search warrant. Inside Denver's home, police found about 10 grams of marijuana and drug paraphernalia, said police Lt. Bruce McNeill.
In all, police recovered about one and a half ounces of marijuana from the package and Denver's home, he said.
Denver's attorney, Randal Roahrig, said the case and charge would be dismissed when the probation is completed. Denver will not have a criminal record, he said.
"It's not like he's getting any special treatment," Roahrig said. "It's a very common resolution to these types of cases."
Denver played the beatnik best friend, Maynard G. Krebs, on television's The Many Loves of Dobie Gillis from 1959 to 1963. He starred in Gilligan's Island from 1964 to 1967.