After graduating college in 2004, Johnston got a job as an engineer for Morgan-Dollar Motorsports in the Camping World Truck Series. Over the following years, Johnston became the engineer for teams such as JTG Daugherty Racing and Evernham Motorsports. In 2011, while doing engineer work for Michael Waltrip Racing, Johnston became the crew chief for the No. 56 team, driven by Martin Truex, Jr. Over the 19 races they were together, the pair got 1 pole, 2 top-5's, and 5 top-10's. The following year, in 2012, Johnston became Truex's full-time crew chief. Truex Jr. and Johnston finished off the season with 1 pole, 19 top-10's, and 7 top-5's. They would finish 11th in the point standings after making the Chase. In 2013, the pair returned to the No. 56 team. Midway through the season at Sonoma, the team won their first race. They would finish of the season with 1 win, 15 top-10's, 7 top-5's. Johnston moved over to the No. 14 team for Stewart-Haas Racing, driven by Tony Stewart. Midway through the season however, after being involved in the death of Kevin Ward, Jr., Tony Stewart gave his seat up to Jeff Burton and Regan Smith for 3 races. Stewart would eventually return to NASCAR at Atlanta later the same season. In 2015, Stewart and Johnston would pair up again. However, this season would prove to be a challenge for Stewart, as he would only be able to get 3 top-10's throughout the season, leading up to a 28th place finish in the points standings. Johnston would again move teams in 2016, this time moving to the No. 42 team at Chip Ganassi Racing, driven by Kyle Larson. At Michigan in the fall, Kyle Larson would get his first win in the Cup Series. The group would end up finishing the season with 1 win, 14 top-10's, and 9 top-5's leading up to a 9th place in the championship standings. Larson and Johnston would start off the 2017 season with 5 top 2's, one of them being a win. Larson would win 3 more times throughout the season, finishing 8th in the championship standings after having a lack of consistency in the playoffs. Johnston would remain as Larson's crew chief in 2018 and 2019. After a winless 2018 season, the two won at Dover International Speedway in the fall. In 2020, Johnston would become the crew chief for Matt Kenseth after Kyle Larson was caught saying a racial slur during a streamed iRacing event.
