United Airlines Flight 82 had been cancelled on Friday and Saturday, data from flight tracking website FlightRadar24 showed, while the airline's website said it had waiver policies in place for passengers travelling to, from or through Delhi until Monday.
UA Flight 82 has resumed operations, but "we will continue to monitor conditions over the next few days", a spokesman said.
The third-largest US carrier is monitoring advisories as the Delhi region remains under a public health emergency, and is coordinating with respective government agencies, the United Airlines spokesman had earlier told Reuters.
Last week, Delhi declared a pollution emergency as toxic smog hung over the city for days, with tourism operators reporting cancellation of bookings for the Christmas holidays.
US rivals Delta Air Lines Inc and American Airlines Group Inc said they do not operate flights to Delhi, while several Asian airlines contacted by Reuters said they had not cancelled flights.
"At the national level, India tops the index rankings (i.e. the country with the worst air quality), followed by Bangladesh and Thailand," said Richard Hewston, Verisk Maplecroft's global head of environment and climate change.
The company's Air Quality Index assesses the atmospheric concentrations of fine particulate matter smaller than 2.5 micrometres, known as PM 2.5.
A US embassy measure of PM 2.5 showed a reading of 481 in New Delhi on Monday morning, local time. The outer limit of "good" air is 50.