On 7 April 1926, Gibson shot Mussolini, Italy's National Fascist Party leader, as he walked among the crowd in the Piazza del Campidoglio in Rome after leaving an assembly of the International Congress of Surgeons, to whom he had delivered a speech on the wonders of modern medicine. Gibson had armed herself with a rock to break Mussolini's car window if necessary, and a Modèle 1892 revolver disguised in a black shawl.

She fired once, but Mussolini moved his head at that moment and the shot hit his nose; she tried again, but the gun misfired. Mussolini's son, in his memoir, gives an alternative account, recounting that Gibson fired twice, once missing and once grazing Mussolini's nose. Gibson was almost lynched on the spot by an angry mob, but police intervened and took her away for questioning.

Mussolini was wounded only slightly, dismissing his injury as "a mere trifle", and after his nose was bandaged he continued his parade on the Capitoline Hill. It has been thought that Gibson was insane at the time of the attack and the idea of assassinating Mussolini was hers and that she worked alone. She told interrogators that she shot Mussolini "to glorify God" who had kindly sent an angel to keep her arm steady.

As she did not hold Irish citizenship due to her Unionist views, she was deported to Britain after being released without charge at the request of Mussolini, an act for which he received the thanks of the British government. The assassination attempt started a wave of popular support for Mussolini, resulting in the passage of pro-Fascist legislation which helped consolidate his control of Italy. She spent the rest of her life in a psychiatric hospital, St Andrew's Hospital in Northampton, despite repeated pleas for her release. She died on 2 May 1956 and was buried in Kingsthorpe Cemetery, Northampton.