Staff Sergeant Mobuto participated in the Fall of Reach and was on the UNSC Pillar of Autumn when it made a blind Slipspace jump to escape from Reach. He was involved with the Battle of New Alexander, where he seen Brutes rip people apart. He participated in the defense of the Autumn and was evacuated before the craft crash landed onto the Halo.
While on the ring, Mobuto was somehow detached from his squad and was contacted by 343 Guilty Spark, who used Halo's teleportation grid to transport them both to the Library, much like he did with John-117. Instructed to retrieve the Activation Index, Mobuto fought valiantly through hordes of Flood Combat Forms, Carrier Forms, Combat Carrier Forms, and Infection Forms. He surprisingly managed to survive until he was in sight of the Index. However, with little experience against the enemy and standard, UNSC-issued ballistic body armor as protection (which 343 Guilty Spark referred to as "insufficient armor"), Mobuto eventually succumbed to the Flood's might. His body was completely thrashed and mangled to the point where even the Flood Infection Forms could not make use of it.
John-117 found what was left of Mobuto's body in the Library during his mission to collect the Index. John discovered Mobuto's identity through his Datapad and paid his respects to the Marine. He was surprised at how far Mobuto had managed to get, as he himself had been pushed to the limit to get as far as he had even with his augmentations and MJOLNIR body armor. Mobuto's determination inspired the SPARTAN to continue through the Flood-infested halls and complete his mission.
- Near his corpse is a damaged ODST helmet. Despite it being unclear how and why it's there, it can be presumed that he came by an abandoned ODST helmet and decided to wear it to keep the "puss" out of his mouth.
- Marvin Motubo's datapads can be found in The Library.