The U.S. leader and First Lady Michelle Obama, along with the couple's daughters, Sasha and Malia, kicked off the festive season in the city by lighting up the decorated blue spruce.
How I Met Your Mother star Harris hosted the event, which attracted 17,000 revellers, and a host of celebrity guests, including Modern Family star Rico Rodriguez, singer Jason Mraz and musician James Taylor.
The political family and its guests led the crowds in a rendition of Christmas classic Santa Claus Is Coming to Town.
Taking to his Twitter.com page at the end of the night, Harris told fans, ""Still can't believe I just hosted the Nat'l Tree Lighting Ceremony. A true once in a lifetime. Fun, too, to stare at the rest of the crowd.""