The 40-year-old actress has portrayed Lily Aldrin on the show since its inception in 2005. How I Met Your Mother follows lead character Ted Mosby (Josh Radnor) as he recalls the events that led to his relationship with his wife to their children, Penny (Lyndsy Fonseca) and Luke (David Henrie). The Mother (Cristin Milioti) was not fully seen on screen until season eight, and her status in "present day" 2030 is unknown.
The show has run for eight successful seasons, but the ninth will be its last. Many fan theories persist that the Mother will be revealed as dead or dying, despite Milioti's claim that the theory is untrue. Others postulate that Ted tells the stories because he knows he has Alzheimer's.
Hannigan did seem to hint at a sad ending when she told Kimmel that she and all of the show's cast and crew did a lot of crying while they filmed the final episode.
"I'd say 99 percent of the room was crying," the actress said. "That's executives and everything."
"The writers really wrapped the show up the way the fans would want it to be wrapped up," Hannigan continued. "They're going to answer all the million questions that everybody has and more."
Show director Pam Fryman has also said that loose ends will be tied up.
"You'll see what happens to everyone," Fryman said. "The last minute or so will really sit with you."
The How I Met Your Mother finale will air at 8 p.m. ET on CBS Monday, March 31.