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To what extent does language production activate amodal conceptual representations? In picture naming, we view specific exemplars of concepts and then name them with a category label, like 'dog'. In contrast, in overt reading, the written word expresses the category (dog), not an exemplar. Here we used a decoding approach with magnetoencephalography to address whether picture naming and overt word reading involve shared representations of semantic categories. This addresses a fundamental question about the modality-generality of conceptual representations and their temporal evolution. Crucially, we do this using a language production task that does not require explicit categorization judgment and that controls for word form properties across semantic categories. We trained our models to classify the animal/tool distinction using MEG data of one modality at each time point and then tested the generalization of those models on the other modality. We obtained evidence for the automatic activation of modality-independent semantic category representations for both pictures and words starting at ~150ms and lasting until about 500ms. The time course of lexical activation was also assessed revealing that semantic category is represented before lexical access for pictures but after lexical access for words. Notably, this earlier activation of semantic category in pictures occurred simultaneously with visual representations. We thus show evidence for the spontaneous activation of modality-independent semantic categories in picture naming and word reading, supporting theories in which amodal conceptual representations exist. Together, these results serve to anchor a more comprehensive spatio-temporal delineation of the semantic feature space during production planning.