Trumpkin is a Red Dwarf who lived in hiding during the Telmarine's occupation of Narnia. In 2303 NT, Trumpkin and two fellow Narnians come across the unconscious Prince Caspian in the woods. After bandaging his head and listening to his story, Trumpkin agrees that Caspian can help the Old Narnians, and aids him in gathering up all the Narnians in hiding for battle. They fight Miraz and his army, but when things go badly, Caspian suggests blowing Queen Susan's magic horn for aid. Trumpkin doesn't believe in the magic horn, the Pevensies or Aslan, and is against relying on it for help, but reluctantly agrees to travel to Cair Paravel in case the supposed help is summoned there.
However, during his travel he is captured by some Telmarines, and is taken to the sea to be drowned. But before this can be done, Trumpkin is saved by the Pevensies, and is very astonished to see them and at first doesn't believe they are the summoned ones at all. But after being bested by Susan in an archery match, Edmund in a sword duel and with Lucy healing his wounds, he accepts them as they are and proceeds to take them to Caspian.
Trumpkin still does not believe in Aslan, however, and is the last to see him when Lucy guides the group across the gorge to the Lion. There, Aslan is playful and tosses Trumpkin up into the air gently, but Trumpkin is terrified and properly chastised for his disbelief.
Once back with Caspian, Trumpkin aids him in overthrowing the traitors who attempted to call up the White Witch, and fights in the second Battle of Beruna.
After Caspian is crowed king, he names Trumpkin to the Most Noble Order of the Lion, and three years later when he sails to the end of the world, he makes Trumpkin his Lord Regent.
In 2356 NT, Trumpkin is very old and half-deaf. He encounters Eustace and Jill near Cair Paravel, but either does not know Eustace or fails to recognize him. He does, however, invite the travelers to stay at Cair Paravel, unknowingly equipping them for their later task of rescuing the lost Prince Rilian.
It is not known how much longer Trumpkin lived after that, but he is present at the Great Reunion in Aslan's Country after the end of Narnia.