Introduction: The COVID-19 pandemic has influenced the daily lives of people and may affect their well-being. The aim of the present study is to assess well-being and associated factors during the first wave of the COVID-19 pandemic in the general population in three European countries. Methods: Green COVID was an observational cross-sectional study using an online survey (7 April 2020 to 24 July 2020) promoted by the Health & Territory Research (HTR) of the University of Seville in Spain, Maynooth University in Ireland, and the University of Winchester in England, which included a sample of 3109 unselected adults. Well-being was measured using the World Health Organization-Five Well-Being Index (WHO-5) scale. Seven aspects, related to the natural environment of the home, were evaluated (role of outdoor views in coping with lockdown, importance of blue spaces during lockdown, importance of green spaces during lockdown, quality of view from home, use of outdoor spaces or window views, elements of nature in the home, and views of green or blue spaces from home). Binary logistic regression was conducted to identify the parameters associated with poor well-being. Results: Mean age was 39.7 years and 79.3% lived in Spain, the majority in urban areas (92.8%). 73.0% were female and 72.0% had undertaken university studies. Poor well-being was reported by 59.0%, while 26.6% indicated the possible presence of clinical depression. The factors most associated with poor well-being were students (OR = 1.541), those who had no engagement in physical activity (OR = 1.389), those who reported ‘living in Spain’ compared to Ireland (OR = 0.724), being female (OR = 1.256), poor quality views from home (OR = 0.887), less benefit from views of the natural environment to cope with lockdown (OR = 0.964), and those younger in age (OR = 0.990). Conclusions: More than half of participants reported poor well-being and one in four indicated the possible presence of clinical depression during the first wave of the COVID-19 pandemic. We identified that belonging to a younger age cohort, being a student, being female, not being able to continue with daily pursuits such as physical activity, and having poorer quality of views from home led to poor well-being among participants. Our study highlights the importance of continued physical activity and views of nature to improve the well-being of individuals during times of crisis such as the COVID-19 pandemic.