Matintalo broke the national age-13 record in the 1000 metres in 2009 with her time of 3:01.01; the following year she set age group records in both the 600 metres (1:34.21) and 1000 metres (2:58.51). In 2011, as a 14-year-old, she won silver in the 800 metres at the national senior championships; she improved her personal best by almost six seconds to 2:05.87, which was a national under-16 and under-18 record and the best time in the world by an under-16 runner that year. Matintalo won silver again in 2012 (2:07.43) before winning her only individual national title in 2013. She represented Finland in the women's 800 metres at the 2012 World Junior Championships in Barcelona, but was eliminated in the heats; at the 2013 World Youth Championships in Donetsk she survived the heats, but went out in the semifinals.