Harold makes his debut in Shrek 2. When his daughter Fiona arrives at the castle with her new husband, he is surprised that they are both ogres. Harold had secretly made a promise to the Fairy Godmother that her son Prince Charming would marry Fiona. Harold hires Puss in Boots to assassinate Shrek. Harold then succumbs to guilt when Fiona finds out that Shrek's not around. Harold at first refuses when the Fairy Godmother asks him to give the love potion to Fiona that will make her fall in love with Charming, because he recoils from this invasion of her free will, but he is forced to acquiesce by some dark threat of disclosure; in the event, when Fiona states that she loved the old Shrek (rather the "new" one that Charming is pretending to be), Harold swaps the cup which contains the potion for his own, thereby consuming the potion himself. At the ball toward the end of the film, Harold protects Fiona and Shrek from a blast of magic from Fairy Godmother's wand. He is finally transformed into his original form of the Frog Prince when he is hit by the Fairy Godmother's magic wand. The wife seems not at all unhappy about this turn of events. After this, he apologizes to Shrek and accepts him into the family, his wife saying that he's more of a man for accepting Shrek and Fiona's marriage than he ever was when he was actually human.
Harold makes a very brief appearance in Shrek the Third as a frog king where he sadly dies of a sickness (it is stated in other materials, such as The Legend of Shrek, a storybook, that the transition between human to frog mixed up his age and accelerated the aging process). He tells Shrek on his deathbed that he and Fiona are next in line for the throne of Far Far Away, and Shrek refuses as he cannot imagine an ogre as king. The king dies after telling Shrek about Fiona's cousin Arthur.
In Shrek Forever After, he appears very briefly in a couple of flashbacks; the first one shows him and Lillian about to make a deal with Rumpelstiltskin, but they break it off at the last moment. In the alternate reality of the second flashback, Harold and Lillian literally disappear after signing over the kingdom to Rumpelstiltskin. At the end of the movie the original timeline is restored, leaving the current time a year or so after Harold's death. However, there is a large painting of him in which the face moves, indicating that his spirit inhabits the picture.
The king's role was a partly reference to another character John Cleese once starred: Basil Fawlty from Fawlty Towers. The king once makes a reference to "his old war wound" to avoid his wife. This is obviously based on Fawlty's character, who often uses his "honorable war wound" (a shrapnel shot from the Korean War) as an excuse to rescue himself from displeasing situations. Also, both Harold and Fawlty are very cynical and sarcastic at times.
It is possible that Fiona's curse was due to the fact Harold was actually a frog.