— The Rankin/Bass character's signature phrase
At the beginning of the original 1969 special, Frosty is built by Karen and her friends right after school has closed for Christmas vacation. The wind blows the top hat of Professor Hinkle onto Frosty's head, and suddenly the snowman comes to life, saying his cheerful catch phrase, "Happy birthday!" Frosty is good-natured and playful, but also somewhat naive due to his relative youth (having just been "born"). He is the world's fastest belly-whopper but, in his first two specials, is unable to count above five.
Frosty has two physical weaknesses: the tendency to melt in warm weather, and the hat, without which he becomes another inanimate snowman. (However, when Frosty gets startled by the Traffic Cop and later by the ticket man, his hat briefly jumps off of his head. Also, in Frosty's Winter Wonderland, he is shown lifting his hat off of his head twice.) Karen and her friends offer to help Frosty get to the one place where he could never melt - the North Pole. However, Professor Hinkle chases after Frosty and Karen, trying to steal his hat back, and eventually traps them in a greenhouse full of Christmas poinsettias when Frosty takes Karen inside to warm up due to how cold she had gotten. Fortunately, Santa Claus manages to revive the melted Frosty by letting in a December wind and afterwards takes him to the North Pole.
Frosty promised to be back again someday, and in Frosty's Winter Wonderland, he fulfills his promise. The character's design is only slightly altered from his original appearance, primarily the addition of a scarf. As much as he enjoys playing with the kids, Frosty becomes lonely when they have to return to their homes. To remedy this, the kids construct a snow-woman, who Frosty names Crystal. She comes to life when Frosty presents her with a bouquet of flowers, a gift of love. Despite the interference of Jack Frost, who attempts to steal Frosty's hat out of jealousy, the couple are wed by a snow parson (and Jack is mollified when asked to serve as best man).
Frosty later appeared in stop-motion animation in the movie Rudolph and Frosty's Christmas in July. His design in this film was largely the same as his Winter Wonderland appearance, except his hat was now colored orange (despite being the same magic hat) and his eyes were re-colored blue. By now, he and Crystal have two snow children, Chilly and Milly (the means by which snow people procreate is ignored; possibly, they simply built their own children out of snow). In this film, Frosty is best friends with Rudolph, who serves as adoptive uncle to the children. Frosty and his family accompany Rudolph to Lilly Loraine's Circus by the Sea in Florida, protected by magic amulets provided by the evil Winterbolt, so they can help Rudolph's friend, Milton the Ice Cream Man, with saving his girlfriend's mother's circus from foreclosure.
The pair generally enjoy their time performing with the circus, in spite of Frosty's worries that they may not make it back to the North Pole by the time the amulets wear off. But when Rudolph is tricked by Scratcher into giving the circus's money to Sam Spangles and his nose's light goes out as a result, Frosty comes to his friend's aide, offering to sacrifice his life in exchange for helping Rudolph and protecting his family by giving up his magic hat to Winterbolt. The evil wizard betrays him, but Rudolph is able to retrieve the hat, along with the circus money. To celebrate, Frosty and Rudolph sing a variation of "We're a Couple of Misfits" (from the original Rudolph the Red-Nosed Reindeer special). Frosty and his family eventually do melt after Winterbolt is destroyed, but they are brought back to life by Jack Frost, and they return to the North Pole with Santa.
Appearances in other Christmas media
The Rankin/Bass version of Frosty made a guest appearance in the comic strip FoxTrot, in a storyline that ran from December 16-21, 1996 (reprinted in the collection Come Closer, Roger, There's a Mosquito on Your Nose). Here, he appears in a dream of Paige's ("You were expecting Freddy Krueger?"), in which he takes her to the Land of Animated Christmas TV Specials to cheer her up when a hard homework assignment puts her through much stress. A later FoxTrot strip, published on December 5, 1998 (reprinted in Think iFruity), has Jason watching the Rankin/Bass special and reacting to it as if he were watching a horror movie.
The Legend of Frosty the Snowman features Bill Fagerbakke as the voice of the Rankin/Bass version of Frosty. (Coincidentally, Bill Fagerbakke had previously voiced a parody of Frosty, played by Patrick Star, in the Merry Nickmas short "Patrick the Snowman" a few years earlier.)
Frosty Returns, an unofficial semi-sequel to the original Rankin/Bass special, features John Goodman as the voice of Frosty, who this time concerns himself with the effects of environmental pollution. Notably, Frosty in this special does not require a magic hat to be alive, even though he still wears a top hat throughout most of Returns (borrowed from his friend Holly in the special).
Another parody version of Frosty, voiced by Clancy Brown, appears in Phineas and Ferb Christmas Vacation!, as a snowman that Perry the Platypus brings to life while on his way to his agency's Christmas party. He says to Perry "Hello, wintertime's fun. Follow me!" and then walks off into the street and gets crushed by an offscreen truck. At the very end of the special, Perry fixes him back up, and he is about to walk into the street again, but this time decides against it when he sees a truck smash into a vegetable stand.
Mentions in other Christmas specials
In real life
Frosty was the theme of Gaylord Palm's ICE! event for 2013.