Mancilla began competing in middle distance events from a young age and was the 800 metres bronze medallist at the 1984 South American Youth Championships in Athletics. She made her global debut at the 1986 World Junior Championships in Athletics, competing in the 800 m and the 400 metres hurdles, in Athletics before going on to win the 800 m silver medal at the 1986 South American Youth Championships in Athletics. At later South American Junior Championships in Athletics, she came fourth in 1987 and 1988, then fifth at the 1989 meet.