As an amateur, Sebastian was a member of the victorious Swedish team at the 2008 European Boys’ Team Championship and twice represented his country at the European Amateur Team Championship. He played US collegiate golf at Coastal Carolina University from 2009 to 2013, winning twice and reaching a career-high amateur world ranking of 51st.

Turning professional in 2013, he progressed through European Tour qualifying school in 2014, playing on both the Challenge Tour and European Tour, winning the 2016 Barclays Kenya Open and later qualifying for his first major, the 2016 U.S. Open. Sebastian’s breakthrough came at the 2019 Omega European Masters, where he triumphed in a five-man playoff that included Rory McIlroy, the win earning him a two-year exemption and lifting him to a career-best world ranking. Seb enjoyed a strong 2024 season, tied third in the Volvo China Open, second in the ISPS HANDA Championship and tied second in the Hero Indian Open. That run of form meant winning the Asian Tour Swing to earn a place in the season’s second Major, the US PGA Championship at Valhalla.