Mutation breeding offers a rapid and effective approach to create novel genetic variation in rice (Oryza sativa L.) and identify mutants with desirable traits. This study evaluated the radiosensitivity of the long-grain, soft, and aromatic inbred rice variety NSIC Rc 218 (Mabango 3). Seeds were exposed to gamma radiation using a Cobalt-60 source, with doses ranging from 100 to 1000 Gy. For all treatments, 1,500 seeds were irradiated and grown under controlled conditions. Seedling vigor and shoot–root development were assessed at 9, 14, and 21 days after sowing. Our results showed a clear dose-dependent response in early growth traits. Low-to-moderate doses (100–300 Gy) produced variable physiological effects, including slight stimulation of shoot growth at 100 and 200 Gy. In contrast, higher doses (≥500 Gy) caused sharp declines in germination and survival. No seedlings survived beyond 600 Gy, indicating the lethal threshold. Overall survival rates ranged from 59% to 77%, with LD50 estimated at 424.11 Gy. Optimal stimulation of early growth traits occurred between 200 and 300 Gy, suggesting this range as a practical mutagenic window for generating genetic variation while maintaining seedling viability. These findings provide essential baseline radiosensitivity data for NSIC Rc 218. More broadly, such studies strengthen the foundation of mutation breeding in rice and other cereals, where optimized irradiation protocols are critical for developing mutants with improved yield, nutritional quality, and stress resilience. By refining dose–response knowledge across crop species, mutation breeding continues to play a key role in addressing global food security challenges.