In this particular case study, we present a 66-year-old male who was diagnosed with an atrial myxoma eight years after receiving treatment for non-small cell lung cancer. The patient underwent chemo-radiotherapy (mediastinal area) in 2012 to address stage III-A adenocarcinoma of the lung. During follow-up imaging in 2020, a left atrial mass displaying characteristic features of a cardiac myxoma was detected. Upon reviewing a computed tomographic (CT) scan from 2017 within the previously irradiated mediastinal region, the cardiac mass was retrospectively identified. The surgical excision of the cardiac mass was performed, and a subsequent pathological examination confirmed the diagnosis of myxoma. To the best of our knowledge, this is the first reported case of a left atrial myxoma in a patient previously treated for adenocarcinoma of the lung and the first instance of an atrial myxoma occurring in a site that had undergone prior radiation therapy.