Fallon had canceled last week's tapings after his mother's death and told the Associated Press through his spokeswoman that he lost his "biggest fan".
"Mom, I'll never stop trying to make you laugh", Fallon said, raising his hand in a squeeze gesture.
According to Tonight Show producer Mike DiCenzo, Swift wasn't scheduled to do the show, and her last-minute piano-accompanied rendition had everyone in tears. It was Fallon's first show back since his mother Gloria passed away, and at the beginning, Fallon reflected on his time with her.
"I grabbed her hand and squeezed, 'I love you, ' and I just knew we were in trouble". She said yes with zero hesitation.
Referring to the line in the song about hand-squeezing, he added, "I almost gasped. I think there was like a thing where everyone wanted to be a nun".
Fallon then became emotional while sharing an anecdote about his mother and the way she expressed her love for Fallon and his sister, also named Gloria.
"I could see Jimmy silhouetted at his desk dabbing his eyes with a tissue", he said. "Whatever you think of Taylor, she did something lovely for Jimmy and our show today, and we're forever grateful". It was a attractive coincidence in a lovely performance.
All our best to Fallon and his family. Thank you to Taylor, the Roots, [Steve Higgins], our whole staff and crew, and to all the FalPals, fans, and everyone who reached out to and supported Jimmy and his family.