Mandarin lexical tones are modified in both infant-directed speech (IDS) and Lombard speech, resulting in tone hyperarticulation. However, it is unclear if these registers also alter contextual tones (neutral tone and tone sandhi) and if such phonetic modification might affect acquisition of these tones. This study therefore examined how neutral tone and tone sandhi are realized in IDS, and how their acoustic manifestations compare with those in Lombard speech, where the communicative needs of listeners differ. Neutral tone and tone sandhi productions were elicited from 15 Mandarin-speaking mothers during (1) interactions with their 12-month-old infants (IDS), (2) in conversation with a Mandarin-speaking adult in a noisy environment (Lombard speech), and (3) in conversation with a Mandarin-speaking adult in a quiet environment (adult-directed speech). The results showed that, although both contextual tones were modified in IDS and Lombard speech, their key tone features were maintained. In addition, IDS and Lombard speech modified these tones differently: IDS increased pitch height and modified pitch contour, while Lombard speech increased pitch height only. The realization of neutral tone and tone sandhi across registers is discussed with reference to listeners' different communicative needs.