The mandate applies to anyone aged 2 or older, regardless of vaccination status. It will go into effect Monday, Pritzker said.
RELATED: Chicago indoor mask mandate takes effect; Cook County to follow suit
The mask mandate will apply to gyms, restaurants, bars, grocery stores, health clubs and other indoor venues.
Pritzker said Illinois is joining other states with its mandate, adding "masks work, period."
WATCH: Gov. Pritzker announces IL mask mandate
Pritzker also announced a vaccine mandate for all education employees for K-12 and higher education statewide. Anyone who does not get a COVID vaccine by Sept. 5 will have to submit for weekly COVID testing.
The Illinois Federation of Teachers welcoming the requirement.
"So I don't think it's going to be controversial with our members we support the mandate. It's a way to make sure school goes on," said Dan Montgomery, President of Illinois Federation of Teachers.
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Pritzker said Thursday that hospitalizations are dramatically up across the state and involve, almost exclusively, those who are unvaccinated.
"You don't need to be an epidemiologist to understand what's going on here, this is a pandemic among the unvaccinated," Governor Pritzker said.
The governor said that from January through July, unvaccinated people accounted for 98% of COVID cases, 96% of hospitalizations and 95% of the deaths related to the virus.
He specifically pointed to what he called "dangerously low ICU bed availability."
Downstate is where hospitals are most stressed.
Rates around Chicago are hovering around the 20% mark, but just 4% of beds are available in downstate Region 5 -- the same region with the worst vaccination rate.
Illinois health officials "fear the worst is yet to come for us," he added.
Illinois is the most vaccinated state in the Midwest, with just over 65% of its population vaccinated, the governor said.
WATCH: Political analyst weighs in on other possible COVID restrictions
"Remember, these vaccines are doing what they're designed to do, essentially to eliminate the risk of hospitalization and death," Pritzker said.
The mask mandate will not be lifted until the hospitalization numbers go down, the governor said.
While the move is something supported by the Illinois Retail Merchants Association, it is also likely to get pushback in some communities less severely impacted by the virus.
"Southern Illinois and Central Illinois have a problem. Well, that's not us. Why are we being forced to put masks on when we don't have a problem," said Herb Otto, from Lemont.
Pritzker said his number one concern is keeping the health care system available, not only for people suffering from COVID, but for others as well.
Pritzker had previously announced a mask mandate for all K-12 schools and daycares to combat the spread of COVID-19 among children, most of whom are not yet approved to receive a COVID vaccine.
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Some school districts, like in Wheeling, already have their own employee vaccine mandate.
"We really are at a fulcrum point in terms of our exposure to children, who are not able to be vaccinated, and their families," said Michael Connolly, superintendent for Wheeling Community Consolidated School District 21.
He hinted Wednesday that new mitigations were coming.
"We're going to look at what we need to do at the state level to expand either mask mandates or vaccine mandates, again, based upon the need to lower the pressure on our hospitals," Pritzker said at the time.
However, as the governor imposed the new mitigations efforts, one thing he said he is not considering at the moment is instituting any capacity restrictions. But some business owners are holding their breath.
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"We're doing everything from the restaurant side to keep everybody safe, our staff and our customer safe. So we're just hoping that we can continue to operate as-is and masks are the only mandate in place," a Manny's Deli employee said.
At Tapville Social, the word of the mask mandate was greeted with disappointment.
"Obviously we'll comply with the mandate because we want to do the right thing as a business, but it does have an impact on a small business where there may be less guests going out with the mandate," said Joseph Tota, owner of Tapville Social.
For couples who have waited more than a year to tie the knot, it could be the last straw.
"At least half would make the decision to move on. We'll get married. We'll have our minister come to our home or stand outside in our backyard," said wedding planner Jane Himmel.
Recently Chicago and Cook County both brought back their indoor mask mandates for the same age groups. There are narrow exceptions, such as removing a mask while eating or drinking in bars, restaurants or other establishments.