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Throughout the COVID-19 pandemic in the US, counties adopted numerous nonpharmaceutical interventions, such as mask mandates and stay-at-home orders, to slow COVID-19 transmission and prevent hospitals from reaching full capacity. Early evidence has been mixed about whether these interventions are effective. However, most studies only covered the early waves of COVID-19 and did not account for county-level variation in the adoption and repeal of such policies. Using daily county-level data from the Centers for Disease Control and Prevention, we evaluated the joint impact of bans on large gatherings, stay-at-home orders, mask mandates, and bar and restaurant closures on slowing COVID-19 transmission during waves 1-4 of the pandemic in the US (March 1, 2020-June 30, 2021). Our survival analysis showed that these interventions were generally effective at slowing COVID-19 transmission during this period. The mitigating effect was particularly strong during waves 2 and 3 and less substantial during waves 1 and 4. We also found strong evidence of the overall protective effect of mask mandates and, to a lesser degree, anticongregation policies. These study findings provide crucial evidence for public health officials to reference for support when using nonpharmaceutical interventions to flatten the curve of future waves of COVID-19 or other infectious disease outbreaks.