Rantanen was used internationally for the first time at the World Games 2005 in the Ruhr area. Rantanen came in seventh place in the individual competition and in eleventh place with the Finnish mixed relay. In 2006 she ran her first season in the World Cup. At the European Championships in 2008 in Ventspils , Latvia , she finished fifth and sixth in the individual competitions as well as second place in the middle distance. Rantanen was just behind her compatriot Heli Jukkola . In the same year in the Czech Republic she made her debut at the World Championships for active athletes. In the middle distance she came in sixth when her teammate Minna Kauppi won . Together with Kauppi and Katri Lindeqvist , she won the relay competition. The Finnish women's relay managed to win the World Cup again in 2010 and 2011. At the European Championships in 2010 and 2012, the Finns also won silver with Rantanen. At the home world championships in 2013 in Vuokatti , Merja Rantanen won her first individual medal at world championships when she finished third in the middle distance behind Swiss Simone Niggli and Swede Tove Alexandersson .
Rantanen runs for the Jämsän Retki-Veikot club . Between 2009 and 2011 she ran for Tampereen Koo-Vee .
