The company's first programme was a chassis for the Trans-Am Series, which achieved considerable success and established Riley & Scott's reputation as a capable constructor. From this base the company moved into sports prototype development.
In 1993 Bill Riley began initial design work on a sports prototype to meet the International Motor Sports Association's World Sports Car regulations, which called for low-cost open-cockpit prototypes to replace expensive GTP cars. The car was redesigned after Dyson Racing contracted Riley & Scott to work on their existing Spice-Ferrari chassis, and the revised Mk III was completed in only four months. Its steel tube frame with carbon fibre panelling, combined with aerodynamics refined using early computational fluid dynamics software and tested at a Lockheed wind tunnel, allowed the design to accept a variety of naturally aspirated engines at a price of approximately $285,000 without an engine. In total 17 Mk IIIs were built from 1995 to 1998, with a further four Series 2 cars produced in 1999.
The Mk III debuted at the 1995 24 Hours of Daytona with Dyson Racing and won five races in its debut IMSA GT season. Doyle Racing's Mk III-Oldsmobile combination won both the 24 Hours of Daytona and the 12 Hours of Sebring in 1996, with Wayne Taylor taking the WSC Drivers Championship. Dyson Racing won the 1997 team championship and Butch Leitzinger the drivers title. By 1998 teams in both North America and Europe were campaigning the car: in the International Sports Racing Series, the French Solution F team of Philippe Gache scored the Mk III's first success outside North America with a win at Kyalami. Over its competitive life the original Mk III accumulated 48 overall victories, 38 pole positions, and 31 fastest laps across 135 races, with 6 teams championships and 8 drivers championships.
Key drivers associated with the car in North America included James Weaver, Butch Leitzinger, and Elliot Forbes-Robinson, who between them drove the majority of the Dyson Racing entries. The 1999 Series 2 Mk III won the 24 Hours of Daytona for the third time in four years. Italian privateer Team Rafanelli campaigned a BMW-powered variant and won the only Mk III victory in the American Le Mans Series at the 1999 Grand Prix of Atlanta.
When Riley & Scott were contracted by General Motors to develop the Cadillac Northstar LMP in 1999, Mk III development paused. Once that project was complete, the company designed an entirely new car — the Mk III Series C — incorporating lessons from the Northstar programme while retaining the steel tube frame and carbon fibre construction of the original. Five Mk III Cs were built. The car debuted at the 2001 12 Hours of Sebring with Dyson Racing and finished fifth in its first outing. The Mk III C, unlike its predecessor, never won a race; it recorded zero victories, poles, or fastest laps across 32 events, with its best results coming from podium finishes in the Rolex Sports Car Series and the ALMS. Dyson Racing elected to return to the older Mk III for parts of 2002, with which they continued winning Rolex Series races. The last Mk III C retired from competition at the end of the 2005 ALMS season with Autocon Motorsports.
Riley & Scott was selected as a chassis supplier for the new Indy Racing League in 1997. The MkV IndyCar was competitive but arrived too late for the 1998 season to attract significant customer orders, with most teams already committed to Dallara or G-Force. A Riley & Scott entry finished twelfth at the 1999 Indianapolis 500. The updated MkVIII replaced it in 2000; Buddy Lazier scored the constructor's sole IRL victory at Phoenix that year. The MkVIII proved effective on short ovals but was uncompetitive at Indianapolis and superspeedways, and Riley & Scott withdrew from the series.
Riley & Scott also assisted in the development of the Chevrolet Corvette C5-R for General Motors and supplied Trans-Am and road car bodies under the Mk I and Mk II designations. The company also developed an IndyCar designated the MkVII.
Reynard Motorsport acquired Riley & Scott in 1999 as part of an expansion strategy. When Reynard went bankrupt in 2001, Riley & Scott was dissolved along with it. Co-founder Bob Riley established Riley Technologies the same year to continue chassis construction, which went on to produce the Riley Daytona Prototype used extensively in the Rolex Sports Car Series through the 2000s.