Arab Muslim armies began their conquest of the Persian Sassanian Empire in ad 636 and during the next five years conquered all of Iran, with the exception of the Elburz Mountains and the Caspian coastal plain. They finally put an end to the Sassanid dynasty in 651. For the next two centuries, most of Iran (which at that time extended beyond Herat in what now is western Afghanistan) remained part of the Arab Islamic empire. The caliphs (successive Islamic leaders) ruled initially from Medina in present-day Saudi Arabia, then from Damascus, Syria, and finally from Baghdad, Iraq, as each city became the seat of the caliphate. Beginning in the late 9th century, however, independent kingdoms arose in eastern Iran; by the mid-11th century, the Arab caliph in Baghdad had lost effective control of virtually all of Iran, although most of the local dynasties continued to recognize his religious authority.

The 270 members of the unicameral legislature—the Majles, or the Islamic Consultative Assembly—are elected for four-year terms; recognized minorities have token representation. The Majles enacts all legislation and is empowered to impeach the president with a two-thirds majority vote. A 12-member Council of Guardians supervises elections and determines the constitutionality of all laws passed by the Majles, as well as their conformity to Islamic principles; six theologians are appointed by the faqih, and six other Islamic jurists are nominated by the High Council of the Judiciary and approved by the Majles. The Committee to Determine the Expediency of the Islamic Order, made up of jurists from the Council of Guardians and selected government officials, resolves disputes between the council and the Majles. The National Security Council oversees the country's policies on defense and security.

11th century Turkic tribes began migrating to Iran, settling primarily in the northwest. The Seljuk Turks (see Seljuks), who had converted to Sunni Islam in the 10th century, defeated local rulers and established dynasties that ruled over most of the country until the Mongol invasions in the 13th century. Mongol rule proved disastrous for Iran. The Mongols destroyed major cities such as Ardabil, Hamadan, Maragheh, Neyshabur, and Qazvin, and they killed almost all of the inhabitants as punishment for resistance. Ray and Tus, the largest and most important cities in Iran, were destroyed by the Mongols and never rebuilt. The Mongols devastated many regions, especially Khorasan and Mazandaran, by destroying irrigation networks and cropland. The harsh rule of the Mongols contributed to a continuing economic decline throughout the 13th century.