He claimed wins at Misano and Adria and another four podiums, finishing third and losing rookie title to Michael Lewis, but overcoming Maïsano by seven points.

[7] During the 2012 off-season Marciello competed in New Zealand-based Toyota Racing Series, taking ninth place in the championship with a win at Hampton Downs.

[11] Marciello achieved his first GP2 victory in the feature race at Spa-Francorchamps after an intense fight with McLaren junior driver Stoffel Vandoorne in the closing stages.

[25] Meanwhile, wins at Budapest and the season finale at the Nürburgring alongside a slew of further podiums gave Marciello and Meadows the Sprint Cup title.

He scored podiums at Bathurst and Spa, as well as taking a win at the Suzuka 10 Hours, the latter earning him the provisional championship lead, though a seventh place at Laguna Seca, a race in which Marciello damaged his car and received a drive-through penalty after a mistimed move on the WRT Audi of Sheldon van der Linde, set him back to third in the standings.

Marciello took part in the FIA GT World Cup, held at the prestigious Guia Circuit in Macau, at the end of the year with Team GruppeM.

[40] In the Sprint Cup, Marciello helped Boguslavskiy towards wins at Misano and Barcelona, though he would finish third in the drivers' standings - one place behind the Russian, who himself ended up mere four points behind the title-winning duo of Dries Vanthoor and Charles Weerts - as he missed the round at Zandvoort due to his commitments at the 24 Hours of Nürburgring.

[45] The team would score two further podiums during the first eight races of the season, which was enough to close the gap to just three points compared to the leading Land-Motorsport outfit of Ricardo Feller and Christopher Mies.

[46] However, a retirement at the Sachsenring owing to a crash caused by Albert Costa and a 30-second penalty which led to a finish outside of the points in Hockenheim put the title out of reach for Marciello and Buhk, who ended the season fourth overall.

[55] In the Sprint Cup, Marciello and Boguslavskiy scored four podiums but failed to win a race, leading to a third-placed finish in the drivers' standings.

Following that round, teammate Jonathan Aberdein was replaced by Lorenzo Ferrari, with personal issues between Marciello and the South African and a lack of pace from the latter being the speculated reasons for said decision.

[58][59] A disappointing event in Zandvoort was the result, though Marciello would bounce back at the Nürburgring, winning the opening race and finishing second on Sunday alongside fellow factory driver Maro Engel and taking the championship lead in the process.

[63][64] Once again, he was joined by Boguslavskiy in the Sprint Cup, where wins at Brands Hatch, Magny-Cours and Zandvoort, along with a heap of fastest laps and pole positions on Marciello's part, enabled the pair to fight for the title, though they would lose out for a third successive year to the WRT duo of Vanthoor and Weerts.

[78][79][80] The Swiss driver was able to stamp his mark on the Sprint Cup campaign as well, which started with a victory apiece at Brands Hatch and Misano, with dominant opening stints from Marciello being the catalysts for success.

[81][82] The duo retired during the first race at Hockenheim, but would bounce back to win on Sunday in spite of gear-shifting issues which manifested during Boguslavskiy's stint.

[83] Another pole position truncated by an imposing first stint from Marciello earned AKKodis victory in Valencia, though this would not be enough to take home the Sprint Cup title, with a collision caused by Audi's Lorenzo Patrese ending the team's final race at Zandvoort, therefore crowning the Tresor Orange1 outfit as champions.

In November 2023, Marciello would drive his final race as a Mercedes-AMG factory driver, competing at the FIA GT World Cup in Macau with Team Landgraf.

[87] A dominant weekend followed, as Marciello took pole, won the qualifying race on Saturday and clinched world cup honours with a faultless drive on Sunday.

[92][93] In January 2024, he was announced to be making his debut in the FIA World Endurance Championship in the top-flight Hypercar class, driving for factory-backed Team WRT in a BMW M Hybrid V8 alongside Dries Vanthoor and Marco Wittmann.