The influences of environmental factors on COVID-19 may not be immediate and could be lagged for days to weeks. This study investigated the choice of lag days for calculating cumulative lag effects of ozone, PM2.5, and five meteorological factors (wind speed, temperature, relative humidity, absolute humidity, and cloud percentages) on COVID-19 new cases at the epicenter of Queens County, New York, before the governor’s executive order on wearing of masks in public places (1 March to 11 April 2020). Daily data for selected air pollutants and meteorological factors were collected from the US EPA Air Quality System, weather observation station of the NOAA National Centers for Environmental Information at John F. Kennedy Airport, and World Weather Online. Negative binomial regression models were applied, including the autocorrelations and trend of the time series, as well as the effective reproductive number as confounders. The effects of ozone, PM2.5, and five meteorological factors were significant on COVID-19 new cases with lag9-lag13 days. Incidence rate ratios (IRRs) were consistent for any lag day choice between lag0 and lag14 days and started fluctuating after lag15 days. Considering moving averages >14 days yielded less reliable variables for summarizing the cumulative lag effects of environmental factors on COVID-19 new cases and considering lag days from 9 to 13 would yield significant findings. Future studies should consider this approach of lag day checks concerning the modeling of COVID-19 progression in relation to meteorological factors and ambient air pollutants.