Having been identified as a potential future Prime Minister by Adnan Menderes, Demirel was elected leader of the Justice Party in 1964 and managed to bring down the government of İsmet İnönü in 1965 despite not being a Member of Parliament. Demirel nominated the Chief of the General Staff, Cevdet Sunay for the presidency in order to soften the army's attitude towards the Justice Party, who became president in 1966.

In the next elections on 10 October 1969, Justice Party was the sole winner by a landslide once again. Demirel presided over the laying the foundations of the Keban Dam, the Bosphorus Bridge and an oil pipeline between Batman and İskenderun. Economic reforms stabilized inflation, and Turkey became one of the fastest growing economies. However boycotts and strikes by university students in 1968 began political instability which especially concerned the Turkish military. Pressure was also mounting from the United States, as the Nixon Administration wished for Turkey to ban the cultivation of opium, which would have been politically costly for Demirel to implement. The army issued a memorandum warning the civilian government in 1971, leading to another coup which resulted in the fall of the Demirel government and the establishment of interim governments.