Just one year later he took his first win in the National Championship. In 2012, Marcus finished fourth in the Rotax Festival Asia, his first appearance in an international event. In 2013, he finished third in the Shifter Kart USA Super Nationals. In 2014, he raced in the New Zealand Formula Ford Championship. In 2015 he continued his karting career before competing in 2016 both in F3 and Formula Renault. Following a trial at the Fiorano Circuit in October, Marcus was taken on by the FDA.

In 2017 he started the season in New Zealand winning three races for M2 Competition in the Toyota Racing Series NZ (Ruapuna, Hampton Downs, Taupo), he then won the Italian F4 series with the Prema Powerteam (winning once at Vallelunga, twice at Mugello and at Imola) and also finished second in the German F4 championship (winning at Spielberg, Oschersleben and Nürburgring). In 2018, Armstrong again raced in the Toyota Racing Series NZ, finishing second with two wins at Ruapuna. He then continued to work with Prema Powerteam making his FIA F3 debut and winning a race at the Norisring circuit.

In 2019 he was runner-up in the Toyota Racing Series NZ with five wins (at Teretonga, twice at Hampton Downs, at Taupo and at Manfeild). He also came second in the FIA Formula 3 championship behind Prema team-mate and fellow FDA student, Robert Shwartzman. He took three wins (Race 2 at the Hungaroring and and Spa-Francorchamps and Race-1 in Sochi). There were a further four podium finishes, one pole position and four fastest race laps.

In 2020, he made his Formula 2 debut, getting on the podium on his maiden outing with ART Grand Prix and finished 14th in the championship. He stays in the F1 feeder series for 2021, switching to the DAMS team.