Mandarin is a representative tonal language with four contrastive tone categories (Tone 1 (T1): high level (ā), Tone 2 (T2): high rising (á), Tone 3 (T3): dipping (ǎ), Tone 4 (T4): high falling (à)). Learning Mandarin tones is known to be difficult for speakers from diverse linguistic backgrounds. The purpose of this research was to examine how native Mongolian-speaking bilinguals perceive Mandarin lexical tones. The 24 (17 females, 7 males) participants studied Mandarin for 15 years on average in the Inner Mongolia Autonomous Region, China. A discrimination experiment was conducted to assess Mongolian bilinguals' perception of six tone pairs (T1-T2, T1-T3, T1-T4, T2-T3, T2-T4, T3-T4). The Mongolian group was less accurate than the control group of ten native Mandarin listeners for all six pairs and the between-group difference was particularly large for T2-T3. However, large individual variation was observed and some Mongolian bilinguals perceived Mandarin tones as accurately as native Mandarin listeners, suggesting that native-like tone perception is attainable in subsequently acquired languages.