The 22-year-old, who is celebrating after her new record Red took the top spot in the UK albums charts, wore a vintage red dress she bought on eBay as she performed three hit singles after she switched on the Christmas lights at Westfield shopping centre in London.
"This is unbelievable. This is incredible. It's unreal," she said.
"I love everything about Christmas. I live on a Christmas tree farm but I never thought I'd be here switching on lights."
Swift, who lives in Nashville, steered clear about talk on the US presidential elections, preferring to discuss her fourth record and her fans.
"I like to say 'hi' to the fans because they're so cute and I love the accents."
She advised potential singers to persist in pursuing their dreams.
"It's important to love it until it becomes all-consuming to your life. Sometimes you can get too ahead of yourself and you give up if it doesn't happen for you quickly," she said.
"Singing should be all you want to do even if you end up singing in a coffee house. That's what I would be doing now if I hadn't had an incredible streak of luck."