Sunset Shimmer first appears as the main antagonist in My Little Pony Equestria Girls. She steals Twilight Sparkle's element of magic crown and during a tussle with Twilight, the crown is thrown into the Crystal Mirror that acts as a portal to another world. She then follows the crown shortly after.
Princess Celestia reveals that Sunset was a former student of hers who began her studies not long before Twilight did. However, when she didn't get what she desired as quickly as she wanted, she turned cruel and dishonest, eventually abandoning her studies to pursue her own path. In this other world, Sunset has been living as a human and student at Canterlot High School for some time. She discovers that Fluttershy found the crown and turned it over to Principal Celestia, much to Sunset's annoyance.
After learning that Twilight will be competing against her for the title of Princess of the Fall Formal, Sunset employs various ways to hinder and humiliate Twilight. She has Snips and Snails record embarrassing videos of Twilight struggling to get used to the human world and releases the videos online, and she doctors photos to frame Twilight for ruining the dance decorations. But Twilight's friends help to counter these ploys by improving Twilight's reputation, and Flash Sentry, Sunset Shimmer's ex-boyfriend, helps prove Twilight's innocence.
At the Fall Formal, when Twilight is named princess and given the crown, Sunset has Snips and Snails kidnap Spike and threatens to destroy the portal to Equestria unless Twilight gives her the crown. Twilight refuses to give the crown up, stating that her being stuck in the human world would be worth stopping Sunset. As Twilight's friends commend her bravery and sacrifice, Sunset snaps and attacks her. Despite Twilight and her friends' efforts, Sunset succeeds in getting the crown and puts it on, and a dark magic aura transforms her into a demon. With her newfound powers, Sunset transforms Snips and Snails into similar-looking minions and brainwashes the other students at the school. She reveals that she never intended to destroy the portal and plans to invade and conquer Equestria with her army of teenagers.
Twilight is able to evoke the powers of the Elements of Harmony when her friends move to protect her from Sunset Shimmer's attack. Temporarily given pony-like characteristics, they use the magic of friendship to defeat Sunset Shimmer and return Snips, Snails, and the rest of the students to normal.
As everyone recovers, a beaten and repentant Sunset is reduced to tears and apologizes to everyone she's hurt. She admits to not knowing anything about friendship, to which Twilight says her friends can teach her. Sunset, Snips, and Snails are later tasked by Vice Principal Luna with rebuilding the part of school they had destroyed during their attack.
Sunset Shimmer re-appears in My Little Pony Equestria Girls: Rainbow Rocks, reformed from her villainous ways in the first film. With the exception of the Rainbooms, the rest of Canterlot High bears resentment toward her, and she even resents herself for the actions she'd taken as what she refers to as a "raging she-demon." She even admits to herself and her friends that she never had a romantic interest in Flash Sentry and only dated him to make herself more popular.
When Sunset gives the Dazzlings a tour of the school, she gets a bad vibe from them when Adagio Dazzle gets defensive about her pendant. Her suspicions are later confirmed when the Dazzlings mesmerize the students and faculty with their singing. Using a magical book with her cutie mark on the cover, Sunset sends an SOS to Princess Celestia in Equestria. During Twilight's role as the Rainbooms' lead singer, Sunset acts in a supportive role and keeps an eye on the competition, especially the Dazzlings during the Battle of the Bands. At one point, Sunset confronts the Dazzlings, who try to manipulate her insecurities by pointing out that the Rainbooms were reluctant to allow Sunset into the band.
When the Rainbooms lash out at each other during the final round of the Battle of the Bands, Sunset realizes what the Dazzlings' true intentions are and is able to stop the petty arguing. She even accepts partial blame due to her lack of knowledge on friendship.
When the Dazzlings overpower the Rainbooms during the film's climax, Sunset Shimmer steps up and sings on the band's behalf. Her help not only gives the Rainbooms the strength they need to defeat the Dazzlings, but she also undergoes a half-pony transformation like her friends. Afterward, Sunset joins the band as a singer and guitarist, and she uses her old journal that she communicated with Princess Celestia through to write her friendship reports to Princess Twilight Sparkle.
Sunset Shimmer is the central focus of the musical short My Past is Not Today, having solo vocals during the titular song. In the song, Sunset sings of how misguided her goals were during the first film and letting go of the past. Likening her reawakening to that of a phoenix, Sunset concludes the song by sprouting fiery magical wings.
Sunset appears throughout the musical short Friendship Through the Ages and has minor vocals during the song. She interacts with the Rainbooms one by one before meeting them in a warm group hug at the end.
Sunset Shimmer appears in My Little Pony Equestria Girls: Friendship Games, competing on the Wondercolts' team in the Friendship Games' motocross event. She is tasked with keeping magic out of the event, but she gets worried that she might fail at understanding how magic works in the human world and that she doesn't have Princess Twilight to help her with her task.
When human Twilight's device starts gathering magic from the main characters and she releases it under pressure from Principal Cinch, she becomes overwhelmed and corrupted by the power. Sunset uses the device to harness the magic of friendship from her friends to stop Twilight's rampage. In the end, Sunset realizes that she is fully capable of solving her own problems and that she can't always count on someone else to do it for her.
The deleted scenes on the DVD reveal a scrapped subplot of the film about Sunset being homesick for her life in Equestria and considering returning.
In the film Legend of Everfree, Sunset Shimmer appears as the secondary central character, serving as moral support for human Twilight. At Camp Everfree, Sunset is paired up with Twilight in the Sapphire Tent. While at the camp, Sunset's magic undergoes a dramatic change, and she develops the ability to sense others' thoughts, memories, and feelings through physical touch.
Throughout the film, Sunset helps Twilight come to grips with the dark magic of Midnight Sparkle inside her, and she guides all of her friends in embracing their new magical abilities. She also helps Flash Sentry get over Princess Twilight, and they start over as friends after their bitter breakup.
When Sunset discovers strange goings-on behind the scenes at Camp Everfree, she initially suspects camp co-director and Twilight's crush Timber Spruce. However, she later discovers it to be Timber's sister Gloriosa Daisy. By helping Twilight overcome her fear of Midnight Sparkle, Sunset proves integral in defeating Gloriosa when she is consumed by Equestrian magic. She and her fellow Rainbooms also take part in saving Camp Everfree from closing down with a Crystal Ball fundraiser.