In the mirror universe, Robert April was a Commodore in Imperial Starfleet, circa mid 2240s to early 2250s. He was married to Sarah April and the two had a very casual relationship as Sarah remarked that she did not care if her husband had extra-marital affairs so long as she did not know about them.

Robert April was asked by Hoshi Sato II to investigate rumors of a new Klingon weapon being constructed. Taking command of the ISS Constellation, April's first act was to execute Nathan Thorpe, Captain of the ISS Indomitable. Captain Thorpe had led the initial expedition to discover the Klingon weapon and had been ambushed, resulting in damage to the Indomitable, and the destruction of the ISS Thermopylae. April was eventually able to discover and locate the Klingon weapon and the system where it was being constructed, the Strelluf star system. Nevertheless, an attack by four Klingon vessals prevented April from bringing the weapon back to the empress. He was forced to destroy both the Klingon base and the Indomitable to prevent the Klingons from making use of the weapon. Later, Sarah accused her husband of becoming soft in his old age, and blackmailed him into subservience by threatening to relate to the empress his failure and disloyalty in securing the weapon. (TOS short story: "Ill Winds")