Objective The objectives of this study were to evaluate the correlation and agreement between ultrasonography and computed tomography (CT) in measuring ascending aorta diameter in patients with giant cell arteritis (GCA) and to investigate the development of new ascending aortic aneurysms in patients with newly diagnosed GCA. Methods To achieve the primary objective, we performed a cross‐sectional study that included patients with GCA who were evaluated with both ultrasonography and CT of the ascending aorta within six months of each other. For the second objective, patients with baseline aortic imaging at GCA diagnosis were observed prospectively with new aortic imaging at the end of the follow‐up. A cutoff of ≥40 mm defined an aneurysm in both the ascending and the descending aorta. Results The cross‐sectional study included 140 patients with a total of 169 comparable CT and ultrasonography scans and a mean disease duration of 47.8 months (95% confidence interval [CI] 42.1–53.5). A strong positive correlation, as measured by Spearman's rho ( r = 0.801, P < 0.001), and a strong agreement, as indicated by an intraclass correlation coefficient of 0.849 ( P < 0.001), were found between ultrasonography and CT. In the prospective follow‐up study, 40 patients were included. Seven (17.5%) patients developed aneurysms in the ascending aorta in a mean follow‐up time of 54 months (95% CI 43.7–64.6); all of the patients experienced large vessel (LV) involvement (two patients with LV‐GCA and five patients with mixed‐GCA). Conclusion A strong correlation and agreement were observed between ultrasonography and CT measurements in assessing the diameter of the ascending aorta in patients with GCA. Ultrasonography may be a valuable tool for longitudinal monitoring and screening for aneurysm development. About 18% of patients developed aortic aneurysms over a mean follow‐up of 4.5 years, underscoring the critical need for regular aortic screening in this population.