This work explored how the effects of input modalities on the production of different L2 contrasts may change over the course of L2 phonological development. Spontaneous imitation and orthography reading tasks were framed within a longitudinal investigation tracking the acquisition of Mandarin stops and lexical tones by Korean learners developing from a near-naïve to a novice stage. Imitation prompted by identical auditory stimuli across different phases revealed a decline in the production of T3, which was produced as T2 to a greater degree with more Mandarin experience. This somewhat counterintuitive finding suggests that the initial benefits from the salient acoustic properties of T3 were overridden by the knowledge of the L2 sound system, namely T3 being realized as T2 via T3 sandhi, leading to diminishing T2-T3 contrasts over time. The stop production data, on the other hand, demonstrated the overarching impact of the existing grapheme-to-phoneme association on the phonological development of new L2 sounds. Some learners prompted with Pinyin produced Mandarin unaspirated stops (e.g., <b d>) with long aspiration and low-F0 for high-level tones. This phonetically unfaithful mapping arose from the familiarity with English voiced stops that are often adapted as lenis stops [long VOT – low F0] by Korean learners. Taken together, the findings demonstrate that the effects of input modalities are specific to the different stages of early L2 phonological acquisition as well as to the types of L2 contrasts.