The COVID‐19 pandemic has radically changed daily life and the way we interact with others. It has also brought negative psychological consequences: several studies have reported increased levels of distress symptomatology at the onset of the pandemic. However, few longitudinal studies have been carried out, and even fewer in low‐ and middle‐income countries. Therefore, the aim of this study was to analyse longitudinally the anxiety and depressive symptomatology of the Argentine general population during the first 13 months of the pandemic. The total sample included 988 adults (ages 18–77) from different regions of Argentina. Open‐access surveys were distributed via social media at five time points (March, April, May, August 2020 and April 2021). Depression and anxiety symptoms were assessed through the Beck Depression Inventory‐II and the State‐Trait Anxiety Inventory. Mixed repeated‐measures ANOVA were applied, with time as a within‐subjects factor, and socioeconomic status, gender, and age group as between‐subjects factors. Results showed a progressive increases in anxiety (F (27.78, 3417.60) = 62.88,
p
< 0.01, ηp
2
= 0.060) and depression (F (3.42, 3373.75) = 84.78,
p
< 0.01, ηp
2
= 0.079) symptoms in the general population throughout the pandemic (except for a slight decrease in anxiety in Wave 2). All of the between‐subjects factors showed significant effects on both types of symptoms: female gender, younger age and lower income were associated with greater depression and anxiety in all waves. A time‐age interaction effect was found (
F
(10.26, 3364.05)
= 2.99,
p
< 0.01,
ηp
2
= 0.009): after the third wave, depressive symptoms tended to decrease or stabilise in the young adult group, whereas there was a positive linear increase in the older groups. This study provided evidence that anxiety and depression symptoms tend to increase during crisis contexts, and that this effect is even more pronounced for certain vulnerable population groups. Mental health monitoring and support interventions should be included in government strategies to deal with the long‐term impact of the pandemic.