Earlier at the president's house, former cricketer and Congress leader Navjot Singh Sidhu met with the Pakistan Army Chief Qamar Bajwa.
Khan, 65, the chairman of the Pakistan Tehreek-e-Insaf (PTI), was administered the oath of office by President Mamnoon Hussain at a simple ceremony held at the Aiwan-e-Sadr (the President House) in Islamabad.
The ceremony commenced with the national anthem, followed by recitation of verses from the Holy Quran. Clad in a black sherwani, Khan was seen little nervous as he faced difficulties in pronouncing some Urdu words during the oath.
Khan's third wife Bushra Maneka was also present at the event. The Oxford-educated Pasthun yesterday defeated his only rival and Pakistan Muslim League-Nawaz chief Shahbaz Sharif in a one-sided election for the top post in the National Assembly.
Khan secured 176 votes while Sharif got 96 votes. A total of 172 votes in the 342-member lower house of parliament are needed to form a government.
This government will be the third consecutive democratic government in Pakistan since 2008 when military ruler Gen Pervez Musharraf announced elections after serving as president from 2001 to 2008 following a bloodless coup in 1999.