In 1945 Third Age he was elected as the thity-second King of Gondor after his predecessor, King Ondoher, was slain in battle. In the previous year a two-pronged attack had been launched upon Gondor by the Haradrim and the Wainriders. In response, King Ondoher led the main army north to counter the Easterling invasion at Dagorlad and was defeated. Eärnil was commander of Gondor's southern army and guarded Pelargir. Eärnil led his army to the Crossings of the Poros were he won a astounding victory over the Haradrim. This enabled him to turn his attention to the invading Wainriders. The Wainriders, certain of victory, were celebrating their success before they would enter Minas Anor. They were unaware of Eärnil's victory in the south and were taken by surprise when Eärnil suddenly attacked them. As a result Eärnil won the Battle of the Camp.
After the threat was eliminated Gondor's attention shifted towards Steward Pelendur and the Council of Gondor, for King Ondoher along with his two sons were slain at Dagorlad. Both he and prince Arvedui of Arthedain (sucessor state of Arnor) claimed the throne. Arvedui based his claim on that he was the direct descendant of Isildur (second High King of Arnor and Gondor and uncle to Meneldil first King of Gondor), and as the husband of Fíriel, only surviving child of King Ondoher. but Eärnil was elected king for his deeds, heritage, and because the Steward, Pelendur, claimed that "The crown and royalty of Gondor belongs solely to the heirs of Meneldil, son of Anárion, to whom Isildur relinquished this realm. In Gondor this heritage is reckoned through the sons only; and we have not heard that the law is otherwise in Arnor". He reigned as Eärnil II.
Aside from being a gifted general Eärnil also was a wise ruler. He sent messages to Arvedui announcing that he received the crown of Gondor, according to the laws and theneeds of the South-Kingdom, "but I do not forget the royalty of Arnor, nor deny our kinship, nor wish that the realms of Elendil be estranged. I will send to your aid when you have need, so far as I am able.". And at last in the autumn of 1973 Third Age, messages came to Gondor that Arthedain was in great trouble, and that the Witch-king of Angmar was preparing a final stroke against it. Then Eärnil sent his son Eärnur north with a fleet, as swiftly as he coud, and with as great a strength as he could spare to aid Arvedui (now king). Eärnur arrived too late to save Arthedain and Arvedui who had fled north (and was drowned in te Ice Bay of Forochel) but manage to defeated the witch-kingdom of Angmar in the Battle of Fornostwith the help of Círdan Lord of Lindon and Glorfindel with an army from Rivendell. There Eärnur earned the hatred of the Witch-king.
However, Eärnil was not able to withstand the takeover of Minas Ithil by the Ringwraiths. Minas Anor was renamed Minas Tirith (Tower of Guard) and Minas Ithil was called Minas Morgul (tower of sorcery).
Eärnil died in 2043 Third Age and was succeeded by his son Eärnur, who was as good a general as his father but lacked his wisdom.