Reactive astrocytes are implicated in amyotrophic lateral sclerosis (ALS), although the mechanisms controlling reactive transformation are unknown. We show that decreased intron retention (IR) is common to human iPSC-derived astrocytes carrying VCP, C9orf72 and SOD1 ALS-causing mutations as well as astrocytes stimulated to undergo reactive transformation. Notably, transcripts with decreased IR and increased expression are overrepresented in reactivity processes including cell-adhesion, stress-response, and immune-activation. We examined translatome sequencing (TRAP-seq) of astrocytes from a SOD1 mouse model, which revealed that transcripts upregulated in translation significantly overlap with transcripts with decreased IR. Using nucleo-cytoplasmic fractionation of VCP astrocytes coupled with mRNA sequencing and proteomics, we identify that decreased IR in nuclear-detained transcripts is associated with increased cytoplasmic expression of genes and proteins encoding regulators of reactivity - indicating nuclear-to-cytoplasmic translocation and translation of spliced reactivity-related transcripts. Our study provides important insights into the molecular regulation of reactive transformation of ALS astrocytes.