Isengard was built over three thousand years ago by the Dúnedain, the Exiles of Númenor, as a fortress on the north-western tip of Gondor. Within Orthanc was placed one of the seven seeing stones, the palantíri.

When Rohan was gifted to the Rohirrim five hundred years ago, the Stewards of Gondor retained Isengard under their own rule, the keys of Orthanc were taken to Minas Tirith, the Tower was shut, and the Ring of Isengard re­mained manned only by an hereditary Gondorian chieftain and his small people. To the northwestern bounds of their land the Rohirrim went seldom and only at need, regarding the eaves of Fangorn (the Entwood) and the frowning walls of Isengard with dread. They meddled little with the "Lord of Isengard" and his secret folk, whom they believed to be dealers in dark magic. And to Isengard the emis­saries from Minas Tirith came ever more seldom, until they ceased; it seemed that amidst their cares the Stewards had forgotten the Tower, though they held the keys.

Over time, the line of the Gondorian chieftains of Isengard failed, and the command of the fortress passed into the hands of a family of the people. They were now more friendly towards the Dunlendings than the Rohirrim, being more closely related to them. In the reign of Déor, King of Rohan, the Dunlendings became openly hostile, raiding the herds and studs of the Rohirrim in Westfold. The king drove them back, but was dismayed to find that Isengard was also hostile. Thinking that he had relieved Isengard of a Dunlendish siege, he sent messengers to its Gates with words of good will, but the Gates were shut upon them and the only answer they got was by bowshot. As was later known, the Dunlendings, having been admitted as friends, had seized the Ring of Isengard, slaying the few survivors of its ancient guards. Déor sent word at once to the Steward in Minas Tirith, but he was unable to send help, and the Dunlendings remained in occupation of Isengard until, reduced by the great famine after the Long Winter they were starved out and capitulated to Fréaláf, a later King of Rohan.

In that same year, Saruman the White, first of the order of Wizards and head of the White Council, offered to take up Isengard as his home. This was welcomed by both the King of Rohan and Steward of Gondor, as Saruman was both wise and powerful. He was gifted the Keys of Orthanc and vowed to command the defences of the West.

In T.A. 2851, the White Council met and Saruman overruled Gandalf when he urged an attack on Dol Guldur. It soon became clear that Saruman desired to possess the One Ring himself. In T.A. 2953, following the last meeting of the White Council, Saruman took Isengard for his own and fortified it. As Lord of Isengard, Saruman began to bring trouble to Rohan by aiding its enemies. Saruman's corruption was complete when, in circa T.A. 3000, he used the palantír of Orthanc and was enticed by Sauron.

Isengard became Saruman's base of operations during the War of the Ring. His war with Rohan began on 25 February, T.A. 3019, with the First Battle of the Fords of Isen. The forces of Isengard were victorious and Saruman's main objective, the killing of Théodred (son of King Théoden), was achieved. With another victory at the Second Battle of the Fords of Isen in early March, the forces of Isengard won passage into the plains of Rohan, but were defeated at the Battle of the Hornburg.

A few days later, Merry and Pippin, now the "doorwardens" of Isengard, welcomed the King of Rohan and his companions. They parleyed with Saruman, but he refused to co-operate. On 22 August, T.A. 3019, Treebeard, now in command of Isengard, allowed Saruman and Gríma to depart from what remained of the fortress.

Following the Battle of Isengard, the Ents destroyed the Ring of Isengard and filled the land with gardens of orchards and trees. A stream ran into a small clear lake, in the middle of which stood the black-stone tower of Orthanc. Whilst the Treegarth of Orthanc became part of the Reunited Kingdom, King Elessar granted the Ents complete self-governance of what was formerly Isengard.