Trummer began his motorsport career in karting back in 2003, finishing second in the Swiss Junior Championship. He also finished 21st in the Andrea Margutti Trophy ICA Junior class.
Formula Lista Junior
Trummer moved up to single-seaters in 2006, competing in Formula Lista Junior. He finished eighteenth overall in the championship with ten points.
Trummer joined the BMS Böhlen Motorsport team in 2007, to compete in the Formule Renault 2.0 Suisse championship. He wasn't so successful in this series, winding up seventh in the championship. Trummer switched teams, joining Jenzer Motorsport for 2008 and finished the season as runner-up.
International Formula Master
After two races at the end of the 2008 season, Trummer moved up to the International Formula Master series for the 2009 season, with Iris Project. He finished in eleventh place in the standings.
Trummer stepped up to the new GP3 Series with Jenzer Motorsport in 2010. He finished the season in 25th place with four points, despite missing the round in Hungary due to driving into the rear of another car in Hockenheim. During 2010 in GP3 with Jenzer Motorsport he was always outperformed by fellow Swiss driver Nico Müller which lead Simon to seak another team for 2011. He moved to MW Arden for the 2011 campaign. Trummer had three point-scoring finishes in final races, improving to 18th position with nine points.
After competing in the non-championship season finale at Yas Marina in 2011, Trummer continued his collaboration with Arden International into 2012 by switching to the GP2 Series full-time. He joined series veteran Luiz Razia in the team, and finished 23rd in the final standings. He then went to Rapax in the 2013 GP2 Series season, and is confirmed with Rapax for the 2014 season.