Sebastián started professionally karting in 2009, arriving with a variety of performances in the categories.
Fernández started his single-seater career in selected rounds of F4 Spanish Championship, two rounds of ADAC Formula 4 and 7 rounds of Italian Formula 4. He finished 14th in Italian Formula 4 Championship, 25th in ADAC Formula 4 and was ineligible to score points as a guest driver in Spanish Formula 4.
For the 2017 racing season, he competed in UAE Formula 4, Italian Formula 4 and ADAC Formula 4. Sebastián raced with Motopark for the UAE championship, driving with Charles Weerts and Leonard Hoogenboom. Here, he won a race and came onto the podium twice, leaving him in 10th place. For the other two Formula 4 championships, he raced with Bhaitech Racing. Since Sebastián was a guest driver in ADAC Formula 4, he was once again ineligible to score points in the championship. He also raced with Bhaitech in the Italian Formula 4 championship, winning 5 races in the season, ending up 4th in the standings.
2018 provided the start for his Formula 3 debut with Motopark (more information below), where he stayed for another year and now in Formula 3 again for the concurrent season.
For 2019, he competed in selected rounds of Formula Renault Eurocup with Arden. Fernández finished on the podium in Spa’s first race. Even though he didn’t race in all rounds of the championship, he scored 68 points, earning 9th in the standings behind Petr Ptáček.
FIA Formula 3 Championship
He made is Formula 3 debut in the European championship during 2018, driving with Motopark for the 30 races. He only managed 5 points, earning 21st in the standings.
When Formula 3 European Championship merged with GP3 to create Formula 3 Championship, he was announced as a driver for Campos, driving the number 25. Sebastián was seated alongside Alex Peroni (until he was injured in the Monza race with David Schumacher as the replacement) and Alessio Deledda. For the championship, he finished 27th, behind Andreas Estner.
During the winter off-season for 2019 and 2020, Fernández competed in the Formula 3 Asian series, driving two rounds with Pinnacle Motorsport. He won a race and stood on the podium 3 times. He finished 8th behind Devlin DeFrancesco.
For the 2020 Formula 3 season, he is driving with Alexander Smolyar and Théo Pourchaire for ART Grand Prix. He will be driving the car number 9.