Born in Finland, Sandelin grew up in Sweden. He developed a love for the game after playing mini-golf in the basement of a McDonald's burger bar in Stockholm in 1977.

He turned professional in 1987 but had to wait for his opportunity on the European Tour, despite attending Qualifying School on a number of occasions. He would then graduate from the Challenge Tour after the 1994 season.

He won the Open De Canaria in his debut European Tour season, eventually finishing 21st on the Order of Merit leading him to being awarded the Rookie of the Year trophy for 1995.

A member of Europe's 1999 Ryder Cup side and five-time European Tour winner, Sandelin waited 18 years between his last European Tour win and first Staysure Tour win, as a seven under par final round saw him claim the MCB Tour Championship - Mauritius in 2019.