It comes as the US reached 50 million confirmed cases of Covid-19 on Monday.
Most deaths have been recorded among the unvaccinated and the elderly, and more Americans died in 2021 than in 2020.
The US is again seeing deaths rising at an alarming rate.
The last 100,000 deaths came in just the past 11 weeks, a quicker pace than any at other point aside from last winter's surge.
"The waves of illness that we're seeing will continue until the population-level immunity is high enough to prevent them. Quite simply, we're not there yet," said Dr Keri Althoff, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.
It has been more than 650 days since the first American patient dying from Covid-19 was reported in Seattle, Washington (public health officials have since attributed earlier deaths to the virus).
Since the Pfizer vaccine - the first jab to be approved in the US - was rolled out last winter, nearly 300,000 more fatalities have been recorded.
In April 2021, two more vaccines - Moderna and single-dose Johnson & Johnson - were approved, and all three vaccines were made available to adults of all ages.
The 800,000 total exceeds the populations of cities such as Boston or Washington DC. The milestone means nearly twice as many Americans have died during the pandemic as in World War 2.
The US death toll far exceeds that of any other country.