Young people suffered as school closures kept them away from friends, and many women found themselves bearing the brunt of household work and facing an increased risk of domestic violence, the researchers said.
The study, led by academics at the University of Queensland, Australia, recorded 76 million additional cases of anxiety disorders and 53 million of major depressive disorder as COVID-19 spread in 2020.
"Sadly, for numerous reasons, women were always more likely to be worse affected by the social and economic consequences of the pandemic," study co-author Alize Ferrari said.
"Additional caring and household responsibilities tend to fall on women, and because women are more likely to be victims of domestic violence, which increased at various stages of the pandemic."
School closures and other curbs limited "young people’s ability to learn and interact with their peers," she added.
The research included 48 previously conducted studies from around the world, and pulled together their findings in a meta-analysis to quantify the prevalence of mental health disorders in 204 countries and territories in 2020.
That made it "the first global insight into the burden of depressive and anxiety disorders during the pandemic," the authors of a linked comment piece who were not involved in the study said.
It found there was an estimated 28 percent increase in cases of major depressive disorder, to 246 million cases, up from an estimated 193 million cases had the pandemic not happened.
There was a similar 26 percent increase in estimated cases of anxiety, with an estimated 374 million cases compared to 298 million without the pandemic.
The authors of the study warned that there was a lack of high-quality data on the impact of the pandemic on mental health in many poorer countries, adding extrapolated estimates for those countries should be interpreted with caution.