Guinevere spent her childhood in Camelot village with Arthur, a stable boy dreaming of kingship. Young Arthur showed her the tree imprisoning Merlin, who prophesied his reign, and pressed a middlemist flower into her hands, promising to fill Camelot with them if she became his queen. When others mocked Arthur's ambitions, Guinevere urged him to follow his heart and seek the prophesied sword.

Years later, Arthur pulled Excalibur from the stone, fulfilling his destiny and becoming king. Though she celebrated with him, Guinevere sensed his distress; he revealed Excalibur was incomplete, missing its blade tip – the Dark One's dagger. Arthur's growing obsession with finding the dagger led him to neglect Guinevere and his duties. On her birthday, he abandoned her celebration to chase a clue. Lancelot comforted her and arranged a gift of rose petals. Discovering Arthur hadn't planned this, Guinevere acted: she retrieved a magical gauntlet from Merlin's tower, an object revealing the dagger's location through the user's greatest weakness. Lancelot accompanied her to the Dark One's vault, where Guinevere saved him from magical darkness using a torch. Relieved, she impulsively kissed him but insisted it couldn't happen again.

Inside the vault, they found the dagger protected by a barrier. Rumplestiltskin appeared, refusing to surrender it but offering enchanted sand from Avalon instead – sand that could create the illusion of fixing broken things, including Excalibur. Despite Lancelot's objections, Guinevere traded the gauntlet for the sand, hoping to mend Arthur's priorities. Confronting Arthur, she threatened to leave unless he abandoned his quest. Fearing she would join Lancelot, Arthur used the sand on her, magically altering her perception to believe their marriage was repaired. Under this enchantment, she became unquestioningly loyal, supporting his schemes. Arthur then used the sand to transform Camelot into a grand kingdom.

Enchanted, Guinevere aided Arthur's manipulation of Emma Swan and her allies, who sought Merlin's help against darkness. She expressed distrust of them during a ball where Percival attacked, leading to his death. She witnessed Arthur's deception in stealing the crimson crown toadstool from David and participated in knightly ceremonies to maintain appearances. When Lancelot reappeared, the enchanted Guinevere vilified him, falsely accusing him of forcing their past kiss. After Arthur was captured in an ambush, she rescued him with knights, used the enchanted sand on David and Mary Margaret, and imprisoned Lancelot. She later observed Arthur's descent into darker methods, like creating a cauldron of molten magic to eliminate enemies; her fearful reaction hinted at underlying unease.

The Dark Curse later transported Camelot's inhabitants to Storybrooke, erasing memories of the previous six weeks. Guinevere reunited with Arthur in the woods but was dismayed by Excalibur's disappearance. She joined efforts to locate the sword and attended town meetings where Arthur revealed the dagger's connection to Excalibur. She supported his attempt to communicate with Merlin using the crimson crown, though Arthur excluded her from the ritual itself. After Arthur's death, she and the other Camelot natives returned to the Enchanted Forest through a portal created by Zelena.