Nunn founded Ensign Racing in the early 1970s and served as both team principal and designer during the team's formative seasons. The Ensign squad entered Formula One in 1973 with backing from pay driver Rikky von Opel and competed in the World Championship through 1982, scoring 19 championship points over the course of 133 grands prix. Nunn's team attracted notable drivers including Chris Amon, Clay Regazzoni, and Derek Daly, and provided Nelson Piquet with his Formula One debut at the 1978 German Grand Prix at Hockenheimring. The team's best result came in 1981 when Marc Surer finished fourth in the Brazilian Grand Prix at Interlagos, also setting the fastest lap in wet conditions.
After 1982, Ensign was absorbed into the Theodore team — which had longstanding financial ties through backer Teddy Yip — bringing Nunn's Formula One chapter to a close.
Following the end of Ensign's F1 programme, Nunn transitioned to American motorsport and established himself as one of the sport's most effective engineers. He joined Chip Ganassi's CART Champ Car operation, where he played a central role in back-to-back championship campaigns. The team won championships with Alex Zanardi and Juan Pablo Montoya in the late 1990s, cementing Nunn's reputation as a top-level engineering mind.
In 2000, Nunn founded his own team — Mo Nunn Racing — to compete in Champ Car. The venture was followed two years later by an expansion into the rival Indy Racing League for the 2002 season, prompted by encouragement from Mercedes-Benz. The IRL team was sponsored by WorldCom and Hollywood cigarettes, and fielded Tony Kanaan and Felipe Giaffone as its lead drivers.
The 2002 Indianapolis 500 brought Mo Nunn Racing close to a landmark result: Kanaan was leading in the final laps before crashing out, while Giaffone finished third and a third car driven by Jeff Simmons crossed the line in ninth place.
The 2002 season demonstrated that the IRL entry was more competitive than the parallel Champ Car campaign, prompting the team to consolidate fully into the IRL from 2003. That year the team ran former Formula One driver Tora Takagi with Toyota power and Pioneer sponsorship. Takagi finished tenth overall while Giaffone missed races due to injury and was replaced by Alex Barron, who took a race victory at Michigan Speedway.
For 2004 the team ran a single car for Takagi, but the programme was hampered by a heavy crash at Twin Ring Motegi that affected the driver's form. Apart from a joint effort with Fernandez Racing at the 2005 Indianapolis 500, Nunn wound down the team during that season and retired from motorsport.
Mo Nunn's contribution to motorsport crossed two eras and two disciplines. In Formula One, Ensign Racing was a tenacious independent constructor that regularly competed against better-funded works teams over a decade, achieving consistent points finishes despite limited resources. In American open-wheel racing, his engineering work with Chip Ganassi's organisation contributed directly to championship victories, while Mo Nunn Racing brought competitive IRL entries and an Indianapolis 500 podium finish. He died on 18 July 2018.