Born in a prominent political family, he grew up destined for civil service. But Michael never wanted this, and preferred more direct approaches to problems than using influence, like he was used to at home. Because of this way of thinking, he eventually ended up with ONI, which he saw as humanity' best shot to recover itself after the war.
In 2556 Horrigan and his fellow squad mates Sedra to investigate terrorist activity. After the biochemical weapon was used by a Sangheili Zealot, he was part of the joint militia/ONI operation to a shard of destroyed Installation 04.
After the Thanolekgolo attack and the destruction of the Condor, it became obvious only two of the people were able to escape the shard with a Tug. Horrigan intended to secure his own survival at any costs and started acting hostile to the two smugglers. When it became clear that Locke was not about to conspire with Horrigan against the others, he eventually made a deal with Ramos. His first act was throwing Haisal Wari downhill and into a group of Thanolekgolo so the others, including himself, could escape.
Eventually Horrigan and Ramos threatened Locke and Aiken and kidnapped Arris Le. The three of them travelled back to the Tug to escape, using Arris to start the biometric system. Eventually Arris escaped and Thanolekgolo were drawn near. This caused Horrigan to betray Ramos by activating the electronics in Ramos' backpack and hitting him in the leg, thus feeding him to the Lekgolo. Horrigan then killed Arris, who had just activated the Tug. Horrigan made a run for the Tug but was quickly swarmed and eaten alive by the Lekgolo drawn to the now active Tug.
- Halo: Nightfall (First appearance)