The One Direction singer was stuck for eight hours at Heathrow airport when he turned up without the document.
Styles called a courier to race to his house to collect the passport, according to Metro.
The 18-year-old was supposed to get on the 11.15am flight but had to wait until 7.30pm.
A source told the Sunday People: "Harry was due to fly out on Friday morning and his mum very kindly offered to drive him to the airport, but when he got there he realised he didn't have his passport…
"He managed to get on the evening flight but it still meant that he had to wait another eight hours for his post-Christmas reunion with Taylor.
"Despite this, he took the whole event in good humour and ridiculed himself for it."
According to reports, the couple, who have been dating since the autumn, are due to spend New Year's Eve in New York.
Swift will perform along with Justin Bieber for Dick Clark's New Year's Rockin' Eve in Times Square.
The stars have been spotted on a trip to the Lake District together and were pictured at a ski resort just before Christmas.