Sarcomatoid carcinoma of the larynx is a rare, highly aggressive malignancy with biphasic epithelial-mesenchymal differentiation. Although its aetiology is multifactorial, prior ionising radiation is a recognised risk factor. In selected patients with differentiated thyroid carcinoma, external-beam radiotherapy (EBRT) may increase the long-term risk of secondary malignancies within the irradiated field. A woman in her late thirties with a remote history of papillary thyroid carcinoma treated with surgery, adjuvant radioiodine ablation and EBRT 13 years earlier presented with persistent hoarseness lasting 6 months. Ultrasound and cross-sectional imaging revealed an infiltrative mass in the right thyroid bed with destruction of laryngeal cartilages, multilevel invasion of the hypopharynx, larynx and tracheal infiltration. Fine-needle aspiration was nondiagnostic, whereas core-needle biopsy confirmed sarcomatoid carcinoma. Serum thyroglobulin remained undetectable/low, supporting a radiation-associated second primary malignancy rather than recurrent differentiated thyroid cancer. Multidisciplinary review deemed the tumour unresectable because of extensive local invasion. Following six cycles of induction chemotherapy with docetaxel and cisplatin, the lesion demonstrated treatment-related regression with a reduction of the central necrotic component and slight tumour shrinkage. Despite persistent locally advanced disease, reassessment at an external head and neck surgery centre led to radical surgery, and the patient subsequently underwent total laryngectomy. This case highlights a rare late complication of EBRT for thyroid cancer. Sarcomatoid carcinoma arising within a previously irradiated field poses significant diagnostic and therapeutic challenges. Long-term vigilance after EBRT is essential, and multidisciplinary management remains critical in this aggressive disease.