He was chosen Prime Minister in a vote at the nation's National Assembly on Friday.
His PTI party won the most seats in July's decisions and Mr Khan has progressed toward becoming PM with the assistance of little gatherings.
Journalists say his need will be to address a money related emergency, with the economy needing an immense bailout.
On Saturday morning, Mr Khan was sorrowful and grinned as he faltered over a portion of the expressions of his vow, driven by the nation's President Mamnoon Hussain.
Individuals from the 1992 cricket World Cup winning group that Mr Khan captained to triumph viewed the function close by senior PTI figures and military authorities.
In Friday's vote, Mr Khan was upheld by 176 individuals. His adversary, Pakistan Muslim League-Nawaz (PML-N) president Shahbaz Sharif, got 96 votes.
In a discourse to parliament, 65-year-old Mr Khan emphasized his battle guarantee to hold "degenerate" legislators to account, and to enhance open doors for youngsters.
"Most importantly, we will begin strict responsibility. I guarantee to my God that everybody who plundered this nation will be made responsible," he said.
"I didn't get on any tyrant's shoulders; I achieved this place in the wake of battling for a long time."
Restriction parties have guaranteed components of a month ago's decisions were fixed. Notwithstanding this, they consented to sit down in the gathering.
Ahead of the pack up to the decision, Mr Khan was broadly observed as the favored hopeful of the great military, which was blamed for interfering to turn assessment against his adversaries.
Before the decision Mr Khan told the BBC that if he somehow managed to be chosen, his underlying spotlight would be on the economy. Pakistan's cash, the rupee, has declined essentially in the most recent year. Swelling is on the ascent and the nation's exchange shortfall is broadening.
Fares, for example, materials have endured a shot from less expensive items made by provincial contenders, including China. Examiners say the new government may need to swing to the International Monetary Fund (IMF) for the nation's second bailout since 2013, which could entangle endeavors to help welfare.