Sebastian Marshall began his rally career in 2005, co-driving for several drivers.

In the 2008 Rallye Deutschland, he made his WRC debut, where he partnered Richard Moore in a Ford Fiesta ST.[2] In January 2015, it was announced he would be co-driving for Kevin Abbring for Hyundai Motorsport in the World Rally Championship.

[3] In the 2017 Rally de Portugal, he replaced Hayden Paddon's then veteran co-driver John Kennard, who was retired from professional career in the previous round.

[5][6] In 2018, the crew was confirmed by the South Korean team to drive the third Hyundai i20 Coupe WRC on some selected rallies.

[7] On 4 December 2018 Marshall was confirmed by Toyota for co-driving for Kris Meeke in 2019.