The coronavirus pandemic (COVID-19) resulted in lockdowns and social distancing measures enforced by governments. Using a cross-sectional design, this study aimed to identify changes in adherence to the Mediterranean diet (MD) and physical activity (PA) and associations with personality, during lockdown in Qatar. A sample of 543 participants was recruited online between April and May 2020. Results showed a reduction in Mediterranean diet (MD) adherence during lockdown (5.9 ± 0.08) compared to before lockdown (6.1 ± 0.08) (p ˂ 0.001). Although there was an increase in the percentage of participants who consumed ≥4 tablespoons olive oil per day (9% vs 12%; p ˂ 0.001), vegetables (54.3% vs 58.7%; p = 0.005), legumes (11.8% vs 15.3%; p = 0.007) and sofrito (70.9% vs 77.3%; p ˂ 0.001), there was also a significant decrease in the percentage of participants who consumed fresh fruit (39.4% vs 15.8%; p ˂ 0.001) and fish/seafood (5.9% vs 3.9%; p = 0.035) and an increase in saturated fat consumption (45.9% vs 53.8%; p ˂ 0.001), during lockdown compared to before lockdown. Participants who scored high in the extraversion personality dimension had a higher MD adherence (B = 0.84; 95% CI = 0.04, 1.64; p = 0.039) before lockdown compared to participants who scored high in the agreeableness personality dimension, although this difference did not reach statistical significance during lockdown. Those who scored high in openness had the lowest change in MD adherence score (B = -0.31; 95% CI = -0.58, -0.04; p = 0.026). Total PA (B = -506.26; 95% CI = -678.60, -333.92; p ˂ 0.001), vigorous activity (B = -155.95; 95% CI = -274.64, - 38.21; p = 0.010), moderate activity (B = -93.04; 95% CI = -148.07, -38.01; p = 0.010) and walking (B = -257.27; 95% CI = -337.87, -176.67; p ˂ 0.001) were decreased during lockdown, while sitting was increased compared to before lockdown (B = 940.91, 95% CI = 831.9, 1049.90; p ˂ 0.001). Openness was positively associated with all PA (B = 562.2; 95% CI = 62.7, 106.7; p = 0.027), including walking (B = 241.7; 95 % CI = 29.4, 454.0; p = 0.026) and negatively associated with sitting (B= -303.4; 95% CI = -590.0; -16.8; p = 0.038) when compared to those with high agreeableness scores, before lockdown. During lockdown, the time spent sitting was lower in those with high scores on neuroticism when compared to those with high agreeableness scores (B= -619.5; 95 % CI = -1215,-23.9; p = 0.042). Differential changes in lifestyle habits by personality dimensions during lockdown, suggest the need to tailor lifestyle interventions based on people’s personality types, for more effective lifestyle change.