The purpose of this study was to assess the effect of COVID-19 lockdown on mood and objective physical activity. A sample of 78 college students in Spain completed an assessment of mood using the valid Profile of Mood State (POMS) questionnaire and had their physical activity tracked objectively using a validated wrist-worn accelerometer (Xiaomi Mi Band 2) for one week before being under COVID-19 lockdown (T1) and for one week during COVID-19 lockdown (T2). Paired samples t-tests revealed significantly greater (p = 0.027) POMS Total Mood Disturbance (TMD) Score T2 (mean ± SD) (22.6 ± 28.0) compared to T1 (17.7 ± 22.6) (lower score represents better mood) and significantly lower (p ≤ 0.001) POMS Vigor Score T2 (14.1 ± 5.0) compared to T1 (18.2 ± 4.5) (lower vigor score represents lower mental and physical energy levels). Additionally, Total Objective Steps was significantly less (p ≤ 0.001) lT2 (15,841.9 ± 17,253.2 steps) compared to T1 (64,607.0 ± 50,525.2 steps). Regression analyses demonstrated significant negative relationships of Total Objective Steps and POMS Depression (p = 0.014, Beta = −0.277, t = −2.511), POMS Anger (p = 0.040, Beta = −0.233, t = −2.091), and POMS TMD (p = 0.007, Beta = −0.302, t = −2.754) T2. The regressions also revealed a significant positive relationship (p = 0.012, Beta = 0.283, t = 2.57) of Total Objective Steps and POMS Vigor T2. These data suggest that being in a lockdown due to a pandemic may have negative physical and mental health-related consequences and that engaging in physical activity may reduce these deleterious mental health-related consequences during lockdowns and quarantines.