Samaia started karting in 2012 before heading straight into racing in 2013 when he competed in the Formula Junior Brazil Championship. By 2015, Guilherme was racing in the Formula 3 Brazil Light Championship where he came out on top with 171 points, where he claimed 13 podiums out of 16 races, 6 of the podiums being wins. He managed to take home 7 fastest laps and two pole positions to finish off his 2015. For 2016, Samaia moved to Formula 3 Brazil where he raced as apart of Cesário F3 in car number 77, alongside team mates Matheus Iorio, Lukas Moraes and Denis Dirani. After a solid season, Samaia managed to take home second with only 65 points between himself and first place, teammate Iorio. Out of 16 races, he managed to claim 10 podiums with 3 wins.
Samaia made his move from Brazilian Formula 3 to British Formula 3 in 2017 where he made his debut with Double R Racing, competing alongside teammate’s Jamie Chadwick, Harry Hayek, Linus Lundqvist, Krishnaraaj Mahadik and Pavan Ravishankar in the BRDC British Formula 3 Championship. Samaia was the only person who competed with Double R Racing who drove in every race of the season and finished 13th with 195 points. The Brazilian also took home his first F3 title in 2017 after winning at the Brazilian F3 Championship.
Heading into 2018, Samaia moved from BRDC British F3 to the Euroformula Open Championship where he competed in the Dallara F312 for RP Motorsport. He was one of 13 drivers to race for the team yet only himself and fellow F2 driver Felipe Drugovich competed in every round. In this season, he raced against old teammate Matheus Iorio. He finished his season in 6th with one podium, securing himself 94 points. He was also involved in the Spanish Formula 3 Championship which was made up of the first two races of the Euroformula season in Portugal and the last four races of the Euroformula in Spain, managing to come out in 5th behind teammate Felipe Drugovich who took home a win in every race with the exclusion of second in the second race of Autodromo do Estoril.
For his 2019 season, Guilherme spent another year in Euroformula but made the move from RP Motorsport to Teo Martin Motorsport. However, his year was cut short after eight races when a contractual dispute with Teo Martin Motorsport erupted. During the eight races, Samaia managed to take a second place finish and a fastest lap.
FIA Formula 2
After a rough start to 2019, Samaia returned to motorsport in the December when he raced alongside Campos Racing in the FIA F2 post-season testing in Abu Dhabi where he was joined by Niko Kari, the 20 year old 2015 SMP F4 Champion. He ended up slowest on two out of the three days that he tested with Campos.
For the 2020 season, Guilherme Samaia will work with Campos Racing as the sixth rookie on the grid. He will be racing alongside teammate Jack Aitken who is returning to Campos for his second year with the team after placing 5th last year.