Siljehaug made his single-seater debut in 2012, racing in Formula Basic and winning the Norwegian Championship in his rookie year. He followed that with the Norwegian Cup title the following year. After a two-year break from racing, he joined Reiter Engineering — the KTM-affiliated team — to contest the GT4 European Series as part of the team's Young Stars programme.
In his first GT4 European Series season with Reiter, Siljehaug took an overall win at Zandvoort and a Pro class victory at Spa-Francorchamps, securing runner-up honors in the points standings. He remained with the team for 2017, taking a lone podium at Zandvoort to finish twelfth in the Silver Cup standings.
After winning the Reiter Young Stars competition in late 2017, Siljehaug continued with the team and made his GT3 debut in the 2018 Blancpain GT Series Endurance Cup, racing a Lamborghini Gallardo R-EX at all events except the 24 Hours of Spa. He scored a Silver Cup podium at Le Castellet alongside Patric Niederhauser and Lennart Marioneck. During 2018 he also made select appearances in the team's GT4 programme, winning twice in the GT4 Central European Cup.
In 2019, Siljehaug ran a dual campaign in the GT4 European Series and the newly-created ADAC GT4 Germany championship. In the European Series he qualified on pole at both Misano and Zandvoort and took a podium at the latter, finishing twelfth in the Silver Cup standings. In ADAC GT4 Germany, he scored victories at Zandvoort and the Sachsenring, added two second-place finishes, and secured the championship title for Felbermayr - Reiter.
Siljehaug switched to the GT4 America SprintX series in 2020 with Marco Polo Motorsports, a fellow KTM-fielding outfit. In his debut season he took overall wins at Sonoma and Road America, plus four additional Silver Cup podiums, to finish fourth in points. He also made a one-off return to ADAC GT4 Germany that year with McLaren-fielding Dörr Motorsport at the Red Bull Ring.
Returning to Reiter Engineering in 2021, Siljehaug raced in select rounds of the 24H GT Series in the GTX class aboard a KTM X-Bow GTX Concept, winning at Mugello, Le Castellet, and Barcelona to finish runner-up in class standings. He also competed in the first three rounds of GT4 America that year for Marco Polo Motorsports, taking a class podium at Sonoma, and made a one-off appearance in Super Super Trucks at Long Beach, taking pole for race two.
Siljehaug moved to LMP3 competition in 2022, joining Freddie Hunt in the Le Mans Cup as Reiter Engineering undertook its first prototype campaign. At the wheel of a Ligier JS P320, he won at the Algarve circuit and scored maximum points in race two at Le Mans, finishing runner-up in the championship standings for the year. Alongside this campaign he raced for the team's True Racing banner in select rounds of the Nürburgring Langstrecken-Serie in the SPX class.
After competing primarily in time attack events in 2023 and 2024, Siljehaug returned to competitive circuit racing in 2025 with Marco Polo Motorsports in the GT4 America Series, driving a Toyota GR Supra GT4 Evo2. He scored Silver Cup podiums at Road America and Barber Motorsports Park and took a pole position at Sonoma, ending the year ninth in points. In 2026, he joined Lamborghini-fielding Chicago Performance and Tuning Co. to make his GT World Challenge America debut in the Pro-Am class.