Born in Bioglio in the Vercelli province of Piedmont, Maglioli began racing in the late 1940s in Italian road races, initially in a Lancia Aprilia. His early career was shaped by his partnership with Giovanni Bracco, who co-drove with him at the Targa Florio and the Mille Miglia across several seasons. Together they finished eighth at the 1948 Targa Florio, taking the T1.5 class victory, and seventeenth at the 1948 Mille Miglia. In 1950, sharing a Ferrari 166 MM Barchetta Touring, they placed fourth overall and won the S2.0 class at the Mille Miglia. Their best joint result came at the 1951 Mille Miglia, where they finished second overall in a Lancia Aurelia B20 while winning the GT2.0 class — Maglioli's best overall position in his ten starts at that race.
In 1953 Maglioli won the Targa Florio outright in a Lancia D20, driving the full event without a co-driver — an exceptional physical feat on the demanding mountain circuit around the Sicilian Madonie. Later that year he and Mike Hawthorn won the Pescara 12-hour race in a Ferrari 375 MM. He made his Formula One World Championship debut at the 1953 Italian Grand Prix at Monza for Scuderia Ferrari, finishing tenth in a Ferrari 553.
In 1954 Maglioli produced his richest season across multiple disciplines. He won the 1000 km Buenos Aires with Giuseppe Farina in a Ferrari 375 MM, then won the Supercortemaggiore at Monza again alongside Hawthorn, and took the Imola Grand Prix in a Ferrari 500 Mondial. In November he won the final edition of the Carrera Panamericana in Mexico, driving the Ferrari 375 Plus across the brutal road stages of the race, which was never held again. In Formula One he contested three rounds for Ferrari: ninth in Argentina, seventh in Switzerland, and a shared third at the Italian Grand Prix at Monza — his F1 podium debut, shared with José Froilán González in the Ferrari 625 with Maglioli driving the first 30 laps and González the final 48.
Maglioli started ten World Championship Grands Prix between 1953 and 1957. He drove for Scuderia Ferrari from 1953 to 1955, for Scuderia Guastalla in a Maserati 250F in 1956, and for Dr Ing F Porsche KG in 1957. His second F1 podium came at the 1955 Argentine Grand Prix at Buenos Aires, another shared drive — he co-drove the Ferrari 625 with Giuseppe Farina and Maurice Trintignant in extreme summer heat. Total championship points scored: 3⅓. His final World Championship start came at the 1957 German Grand Prix at the Nürburgring Nordschleife, driving a Porsche 550RS entered under Formula Two regulations; he retired after 13 laps with engine failure.
Maglioli's move toward Porsche from 1956 yielded an immediate Targa Florio victory — his second — in the Porsche 550 RS, again as a solo driver. Later in 1957, competing at the Gaisburg hillclimb in Austria, he crashed heavily. The leg injuries were severe enough that medical staff initially doubted he would walk without difficulty again. After months of recovery in a Salzburg hospital Maglioli returned to competition, demonstrating the determination that characterised his career across two decades.
Maglioli remained a fixture in endurance racing through the 1960s. He contested the 24 Hours of Le Mans ten times; his first finish came in 1963 when he and Mike Parkes placed third overall in a Ferrari 250P. The following year he and Parkes won the 12 Hours of Sebring in a Ferrari 275 P. He continued at the Targa Florio in almost every season, including class victories with Porsche and a second overall in 1969 with Vic Elford in a Porsche 908/2. His third and final Targa Florio overall victory came in 1968, co-driving a works Porsche 907 with Elford — fifteen years after his first and across three separate decades of competition at the race.
Maglioli made his final Targa Florio appearance in 1970, sharing an Alfa Romeo T33/3 with Nanni Galli. He retired from racing that year. He died in Monza on 7 February 1999.