Manthor was the son of Agathor and Meleth. He ran for Chieftain of the Men of Brethil against Hardang, because Manthor was related to Brandir the Lame, the previous chieftain of the Haladin, and Manthor thought that he could win by his noble blood being that Brandir was his second-cousin. However, Manthor was related to Brandir through Halmir's daughter Hiril, who was the grandmother of Manthor and the sister of Halmir's son Haldir, who in turn was the grandfather of Brandir. This meant that Manthor's kinship with Brandir was worth less in the eyes of the people of Haleth, and he lost the election. And Hardang too was related to Brandir, being that Brandir also was his second-cousin through Halmir's other son Hundar, the brother of Haldir and the grandfather of Hardang. Hence, Hardang was the victor in the election, and he appointed Manthor to be the Captain of the Guard at the Crossings of Taeglin. Sagroth, Avranc, and Forhend were some who were under his authority.
In the Hall, Hardang tried to attack Manthor, because they were of different houses. Húrin managed to wound Hardang in his household, and after an imprisonment was forced to stand trial for it, at the 301st Folkmoot of Judgment. Manthor successfully defended Húrin there, and revealed Hardang's corrupted loyalty with the people, which led to a revolt.
Manthor died in woods next to Nen Girith, by an arrow-wound from Avranc who earlier had turned against him and broken his command. This took place after the revolt caused by Húrin's trial, during which Avranc had tried his second and third time to slay Manthor with an arrow.