Maize, as the global highest-yield grain crop, can impact social stability and security based on its annual yield. Given that maize viruses have caused up to 91% yield reductions, investigating maize virus diseases is of the utmost importance. In July 2020, a suspected maize yellow mosaic virus (MaYMV) was discovered in a maize field, and a MaYMV detection protocol was established. The MaYMV isolate MA70, discovered in a maize plant from Wuri District, Taiwan, in November 2022, was shown to infect both maize 42 days post-inoculation (dpi) and wheat (35 dpi), causing mosaic symptoms, through aphid transmission with corn leaf aphid (Rhopalosiphum maidis). To determine the whole genome sequence of MA70, a 5642 bp sequence was obtained using RT-PCR and Sanger sequencing. Sequencing results indicated a 94.8–96.8% nucleotide sequence similarity with 54 MaYMV isolates from GenBank and with amino acid sequence identities exceeding 90% for all MaYMV proteins. Phylogenetic analysis showed the relationship of MA70 is closest to the Chinese isolate. The nucleotide sequence identity was lower among isolates of more distinct geographical clusters. Between October 2023 and January 2024, survey results indicated that MaYMV prevalence in corn fields across six areas in Taichung reached 17.5% (130/743 plants) and was present in all the sampled fields. MaYMV was present in all sampled fields affirming its ubiquitous presence. This study establishes the first documented case of MaYMV in Taiwan; however, survey findings hint at a potential pre-existing presence in Taiwanese maize fields. Therefore, this research also develops a practical diagnostic tool for field monitoring of MaYMV prevalence, which is crucial for informing future disease management strategies, including the critical need for cross-strait between Taiwan and China collaboration on viral disease surveillance.