When he enters the story, he is one of the "Old Narnian" underground, a network of dwarves, fauns, centaurs, talking beasts and others who are hiding and surviving in inaccessible wooded and mountainous country, to escape harassment from the Telmarine usurpers of Narnia.

He and his allies support Caspian in rousing a hodge-podge force of Old Narnians to mount a resistance to his Uncle Miraz's rule and to challenge the broader repression of Old Narnia.

Despite the success of the horn's call, Trumpkin is true-to-character, singularly unimpressed with the results; four children, when the best he would have imagined are four powerful adult warriors and strategists.

Half a century later, in The Silver Chair, Trumpkin is a very elderly, deaf dwarf who is again running the country while an elderly Caspian is away on his ship to "see again the places of his youth", although many believe that he had set out to seek Aslan to find out who could be the next King of Narnia after him, as his only son Rilian had disappeared 10 years before - not knowing that Eustace and his friend Jill Pole would be arriving in Narnia to rescue and deliver the heir to the Narnian throne.

Glimfeather the owl brings Eustace Scrubb and Jill Pole to Trumpkin, who gives them lodging in the castle of Cair Paravel, but puts no effort into listening to their pleas or getting others to do so.