Lynn competed in the GP3 Series in 2014 for Carlin Motorsport as a member of the Red Bull Junior Team programme. He won on his debut at the Circuit de Catalunya supporting the 2014 Spanish Grand Prix, starting from pole and setting the fastest race lap to take the maximum points from the race. In the sprint race he finished 18th, but still led the championship after its first round. Lynn won again at the Red Bull Ring in Austria, setting pole position before securing a comfortable win over his teammate, Emil Bernstorff. But again he was out of luck in the reverse grid race two, finishing 20th after picking up a lap-one puncture. The Briton was disappointed to miss out on a home win in the third round at Silverstone in the UK, but second in race one and a fighting sixth in race two helped him to maintain his championship lead. In round four at Hockenheim in Germany, Lynn consolidated that lead by finishing second and third in the two races. He then recorded two fourth-placed finishes at the Hungaroring to extend his title advantage to 31 points over Richie Stanaway heading into the summer break. In Belgium, he managed to score 4 championship points to take 8th place and the reverse grid pole for the following sprint race. He dominated the race, leading from start to finish. A similar situation happened in the next round at Monza, Italy where Lynn finished in 6th place for the feature race and managed to get 2nd in the sprint race due to his excellent driving as well as help with the reverse grid order rule in all GP categories.