Members of the Kennedy family were in attendance at the 3:55 ceremony, including his wife, Vicki, his sons, Teddy Jr. and Patrick, a Rhode Island congressman, his daughter Cara and stepson.
Kirk's wife, Gail, broke down in tears when her husband took the oath, followed by a standing ovation in the Chamber.
The galleries were packed with onlookers, as House Speaker Nancy Pelosi, D-CA, took time to attend the ceremony, warmly embracing Kirk with a "congratulations."
More than a dozen senators attended the ceremony, despite it being a day of no votes heading into a weekend, including: Sen. John McCain, R-Ariz; Senate Appropriations Committee Chairman Dan Inouyr, D-Hawaii, his committee counterpart Thad Cochran, R-Miss.; Minority Leader Mitch McConnell, R-Ken.; Sen Ted Kaufman, D-Del., himself a former Senate aide-turned-temporary-replacement (for VP Biden), as well as Mass. now-senior Sen. John Kerry, D-Mass.
Former and current Kennedy staffers ringed the outer-edge of the chamber, as they watched a colleague replace their boss in the August chamber.