The 166 S shared its Aurelio Lampredi-designed tube frame and double wishbone and live axle suspension with the 125 S, retaining that car's 2,420 mm wheelbase. Nine of the 12 cars were built as Spyder Corsas with cycle-fenders, and three as Sports models. Coachwork on the first two examples was supplied by Carrozzeria Allemano, while the third was bodied by Carlo Anderloni at Carrozzeria Touring. The majority of the closely related 166 MM cars were bodied at Touring in barchetta form.
The fundamental mechanical change over the 125 S concerned the engine. Gioacchino Colombo's original 1.5-litre V12 was enlarged to 2.0 litres (1,995 cc) through increases in both bore and stroke, to 60 by 58.8 mm respectively, while single overhead camshafts replaced the twin-cam arrangement. With three carburetors, output ranged from 110 PS (81 kW) at 5,600 rpm to 130 PS (96 kW) at 6,500 rpm, giving a top speed between 170 and 215 km/h. In the 166 MM variant power rose to 140 PS (103 kW) at 6,600 rpm and top speed to 220 km/h.
The 166 S formula was also adapted for road use as the 166 Inter. Both the 166 S and 166 MM were eventually succeeded by the 2.3-litre 195 S.
The Ferrari 166 S inaugurated Ferrari's run of major international victories. In 1948, Clemente Biondetti and Igor Troubetzkoy drove a 166 S to victory at the Targa Florio. That same year Biondetti and Giuseppe Navone won the Mille Miglia in the 166 S. Biondetti returned in 1949 to win the Targa Florio again, this time in the 166 SC variant with Benedetti as co-driver.
The 166 MM extended the winning streak dramatically. At the 1949 Mille Miglia, 166 MM Barchettas scored a 1-2 finish with Biondetti and Salani first, and Bonetto and Carpani second. Later in 1949, Luigi Chinetti and Lord Selsdon won the 24 Hours of Le Mans in a 166 MM, making the 166 the only car ever to win the Targa Florio, the Mille Miglia, and Le Mans within the same period. A 166 also won the 1949 Spa 24 Hours. A 166 chassis fitted with the larger 195 S engine won the Mille Miglia again in 1950, driven by Giannino Marzotto and Marco Crosara.
Motor Trend Classic ranked the 166 MM Barchetta sixth on their list of the ten greatest Ferraris of all time.
The 166 MM, named for the Mille Miglia, was the more numerous and successful companion to the 166 S. Ferrari produced 47 166 MM cars between 1948 and 1953, compared with only 12 of the base 166 S. The increased power output and lighter barchetta bodywork made it the more competitive racing tool in period.
The oldest Ferrari with an undisputed pedigree is chassis number 002C, a 166 Spider Corsa originally built as a 159, currently owned and driven by James Glickenhaus. Chassis 0052M, a 1950 166 MM Touring Barchetta, was discovered in a barn and shown publicly for the first time since 1959 in the August 2006 issue of Cavallino magazine. A further notable survivor, chassis 0018M from 1949, was bodied by Zagato in a Panoramica style closely resembling their one-off Maserati A6 1500 and is considered the first Ferrari coachbuilt by Zagato. It was later rebodied as a Zagato Spyder, and the original body was recreated in 2007 under Zagato's Sanction Lost programme.
The 166 S and 166 MM represented Ferrari's proof of concept on the international stage. Their rapid accumulation of victories at the most prestigious road races of the era transformed Ferrari from a fledgling manufacturer into a credible world-class competitor within the first two years of production. The Colombo V12 architecture refined through the 166 lineage formed the engineering foundation for Ferrari's subsequent road and racing cars throughout the 1950s.