mRNAs carry two layers of information, the genetic code and the information that dictates their post-transcriptional fate. The latter function relies on a complex interplay between cis-elements and trans-regulators, and unbiased identification of these elements is still challenging. To identify cis-elements that control gene expression, we use dimethyl sulfate (DMS) mutational profiling with sequencing and map changes in mRNA secondary structure following viral infection. Our dynamic structural data reveal a major role for ribosomes in unwinding secondary structures, which is further supported by the relationship we uncover between structure and translation efficiency. Moreover, our analysis revealed dozens of regions in viral and cellular mRNAs that exhibit changes in secondary structure. In-depth analysis of these regions reveals cis-elements in 3′ UTRs that regulate mRNA stability and elements within coding sequences that control translation. Overall, our study demonstrates how mapping dynamic changes in mRNA structure allows unbiased identification of functional regulatory elements.