Gríma was secretly in thrall to Saruman, and worked to weaken Théoden and his kingdom. Upon Gandalf's arrival, "many things which men had missed" were found locked in his trunk and he was given a grim choice: ride into battle or into exile. Choosing the latter, he went to dwell with Saruman at Orthanc. Saruman had cause to regret this when, following the confrontation between Saruman and Gandalf, he threw the palantír of Orthanc at the Rohirrim accompanying Gandalf, an act for which Saruman seems to have punished him severely.
He then accompanied Saruman to the Shire, where the two sought revenge in petty tyranny over the hobbits (though Saruman had already been exerting control from afar by sending evil Men to the Shire). During this time he became increasingly degraded until he was a crawling wretch, almost resembling Gollum, and Saruman shortened his nickname to "Worm". During this time he killed Lotho Sackville-Baggins, and may have eaten him.
Pushed into exile again after Frodo's return, he stabbed Saruman to death and was felled by a Hobbit arrow for his treachery.
In Peter Jackson's film The Lord of the Rings: The Two Towers Gríma is played by Brad Dourif. The reason for Gríma's pale and emaciated appearance in the movie is not entirely clear. Perhaps it is meant to suggest that by throwing in his lot with Saruman he has started down the same path to physical and mental corruption that caused Gollum to become a twisted parody of his original self, although it is just as likely that Jackson simply wanted to make it clear that Wormtongue was one of the "bad guys" in the large and confusing cast of characters.