Introduction Thoracic endovascular aortic repair (TEVAR) has become an accepted treatment for thoracic aortic disease. However, the principal complications relate to coverage of the thoracic aortic wall and deliberate occlusion of aortic branches over a potentially long segment. Complications include risk of stroke, spinal cord ischaemia (SCI) and arterial insufficiency to the left arm (left arm ischaemia (LAI)). This study specifically scrutinised the development of SCI and LAI after TEVAR for interventions for thoracic aortic disease from 1999 to 2020. In particular, those who underwent extra-anatomical bypass (both immediate and late) were compared to the length of thoracic aortic coverage by the stent graft. Materials and methods Ninety-eight patients underwent TEVAR. The presenting symptoms, pathology, procedural and follow-up data were collected prospectively with particular evidence of stroke, SCI and LAI both immediate onset and after 48 h of graft placement. Results Fifty underwent TEVAR for an aneurysm (thoracoabdominal aortic aneurysm), 22 for dissection, 19 for acute transection and 7 for intramural haematoma/pseudoaneurysm of the thoracic aorta. Twenty-nine (30%) required a debranching procedure to increase the proximal landing zone (1 aorto-carotid subclavian bypass, 10 carotid/carotid subclavian bypass and 18 carotid/subclavian bypass). Ten patients (10%) died within 30 days of TEVAR. Twenty-four grafts covered the left subclavian artery origin without a carotid/subclavian bypass. Five required a delayed carotid/subclavian bypass for LAI (4) and SCI (1). Six developed immediate signs of SCI after TEVAR and these 11 (group i) had a mean (SD) length of coverage of the thoracic aorta of 30.2 (10.6) cm compared to 21.5 (11.2) cm (group g) in those who had no LAI or SCI post TEVAR, p < 0.05. Conclusions In this series, delayed carotid/subclavian bypass may be required for chronic arm ischaemia and less so for SCI. The length of coverage of thoracic aorta during TEVAR is a factor in the development of delayed SCI and LAI occurrence. Carotid subclavian bypass is required for certain patients undergoing TEVAR (particularly if greater than 20 cm of thoracic aorta is covered).