Dorthinon arrived to Middle-Earth as the sixth and last of the Istari. He was not as great and mighty as the other wizards, but he still possesed formidable powers. He arrived 3 years after the Five, on a lonely boat. He was exhausted because of the heavy storms that caught him while he was traveling through the Great Sea. He arrived at Mithlond after 5 days of suffering. Círdan helped him to recover, and advised him about his tasks.
Dorthinon became good friend of Gandalf and Elrond, and he frequently visited Imladris as personal guest of its lord and to give his reports to the White Council. He got the task to watch over Bree and the nearby villages and root out the spies of the Dark Forces. 
After the return of Sauron, Dorthinon's work became much more dangerous. He reported to the White Council about the growing shadow around Bree and the Shire. After Gandalf found out, that the One Ring is in the Shire, and gave Frodo the task, to be the Ringbearer, Dorthinon got new tasks. As several good warriors of the North has gone, to aid Frodo, so now the Red Wizard must protect the whole region of Eriador, and aiding the northern lands.