Acquired brain injury (ABI), mainly stroke and traumatic brain injury (TBI), is the leading cause of disability in adults. The sequelae of ABI can persist years after the injury (chronic phase), with subsequent burden for the health system and at personal, family, social and economic levels. The most common sequelae affect the cognitive, communicative, motor, and sensory areas. There is solid evidence for the application of multidisciplinary neurorehabilitation treatments, but their effectiveness could be enhanced when combined with additional strategies. Research in animal models has shown that physical exercise can exert neuroprotective and neuroreparative effects and improve cognitive function after an ABI. In addition, both in the general population and in people with cognitive deficits, there seems to be an association between the amount of physical activity in daily life and cognitive function. Therefore, exercise and physical activity could contribute to cognitive rehabilitation in people with ABI. The present work contains two studies. The first study examined the evolution of cognitive function and quality of life over a year, both before and after a supervised intervention with aerobic physical exercise, in a sample of adults with severe TBI in the chronic phase. The second study examined the association between, on the one hand, the amount of physical activity and sedentary time, and, on the other hand, cognitive function, quality of life, and motor function in people with stroke. In addition, cognitive telerehabilitation using the NeuronUP platform was administered for 24 weeks. In the first study, the supervised physical exercise program was associated with an increase in physical activity in daily life (recorded by accelerometry). Similarly to previous data in people with subacute TBI, very low correlations were found between subjective (Borg scale) and objective (percent heart rate reserve) measures of exercise intensity. The second study was carried out with a sample of patients with stroke in the chronic phase during the Covid-19 pandemic, so it was not possible to apply any face-to-face intervention. Significant positive correlations were found between the level of physical activity (by accelerometry) and the performance of several cognitive tests. However, this association was only significant in younger patients (less than 60 years old). In addition, the direction of the association was opposite in men and women (surprisingly, in the latter several cognitive functions correlated positively with a higher proportion of sedentary time compared to walking time). The level of adherence to telerehabilitation was high, particularly during the first 12 weeks of intervention and in those participants who had been receiving face-to-face rehabilitation before the pandemic. Subjective estimation of sedentary time and physical activity (using a questionnaire) showed a very poor correspondence with the accelerometric recordings. In both studies, the patients experienced improvements in some cognitive functions, although the design does not allow these improvements to be attributed to the interventions that were applied. These data give support to the notion that implementing programs to promote physical activity and reduce sedentary lifestyles could potentiate the benefits of cognitive rehabilitation in people with chronic ABI. However, putative sex/gender or age differences (among other factors) need to be thoroughly investigated to tailor these programs more individually based on such differences. Program design must also take into account that self-reported measures of exercise and physical activity may not be adequate for people with cognitive impairment associated with ABI.