Marciello was born in Switzerland and raced in Italian junior series from a young age. He won the opening race of his debut Formula Abarth season in 2010, earning a place in the Ferrari Driver Academy alongside fellow champion Brandon Maisano. He moved to Italian Formula Three with Prema Powerteam in 2011 and worked upward through the European F3 ranks, finishing runner-up in both the FIA European F3 championship and the Formula 3 Euro Series in 2012 — scoring more race wins in each than any other competitor — before returning to claim the FIA European Formula Three title in 2013, winning 13 races and clinching the championship with a round to spare.
The Ferrari Driver Academy placed Marciello in GP2 for 2014 with Racing Engineering. His season was marked by inconsistency — errors and incidents limited him to points finishes on just six occasions — but a victory in a wet feature race at Spa-Francorchamps after an intense late battle with Stoffel Vandoorne, along with three further podiums, placed him eighth in the standings.
For 2015 Marciello joined Trident Racing. He qualified outside the top ten only once across the season, took the same number of podiums as in his rookie year, and finished seventh overall while contributing 99.1 percent of his team's total points. His final GP2 campaign came with Russian Time in 2016, a season dominated by Prema Racing. Through consistent performances and six podiums, Marciello finished fourth in the standings.
On 26 November 2014 Marciello drove the Ferrari F14 T at the post-season Yas Marina test, setting the second-fastest time of the session half a second behind Pascal Wehrlein in the Mercedes. He was announced as a test and reserve driver for Sauber for the 2015 season, but in January 2016 he was dropped by both Sauber and the Ferrari Driver Academy for personal reasons before ever entering a race weekend.
Marciello entered GT racing in 2017 with AKKA ASP in the Blancpain GT Series, driving a Mercedes-AMG GT3. He was awarded factory driver status by Mercedes-AMG ahead of 2018 and swiftly established himself as one of the fastest GT drivers in the world. Highlights of his Mercedes years include the 2018 Blancpain Sprint Cup title, the 2019 FIA GT World Cup at Macau, the 2022 Spa 24 Hours victory — his first major endurance win — the 2022 GT World Challenge Europe overall title, and back-to-back GTWCE Endurance Cup championships in 2022 and 2023. He also took the 2022 ADAC GT Masters title.
After departing Mercedes at the end of 2023, Marciello presented his second Macau GT World Cup victory that November as a "last gift" to the brand. He signed with BMW as a factory driver and joined the WEC Hypercar class with Team WRT for 2024, driving a BMW M Hybrid V8 alongside Dries Vanthoor and Marco Wittmann. His BMW WEC campaign produced its standout result at Fuji 2024, where the crew finished second — BMW's first Hypercar podium in the WEC. In 2025 Marciello took a further win at the 24 Hours of Nürburgring with ROWE Racing.
Marciello's trajectory from the Ferrari junior pathway through GP2 to world-class GT status illustrates how the discipline can absorb drivers who do not reach Formula One. His dominance in the GT World Challenge Europe, multiple Macau GT World Cup victories, and Spa 24 Hours win place him among the elite GT drivers of his generation.