Indoor social gatherings are prohibited and outdoor gatherings will be limited to five people.
"This is a top priority", he says.
Ford says the province is facing a "very serious" situation and this is "a new pandemic" because of variants.
"Small businesses are exhausted of being a scapegoat for the Ontario government's lack of planning or foresight", said Ryan Mallough, CFIB director of provincial affairs for Ontario, in a statement.
Earlier this week, the president of Ontario's Hospital Association (OHA), said hospitals reached a record high with at least 421 COVID-19 patients now in the province's intensive care units.
The per cent positivity of COVID-19 tests in Windsor-Essex increased to 3.6 per cent for last week, compared to 2.8 per cent the previous week.
The capacity limit for weddings, funerals and other religious services indoors is 15 per cent, while outside can be the number of people that can maintain two metres physical distance.
Ontario Health Minister Christine Elliott, meanwhile, said that the dosing interval could be reduced if incoming supply levels allow.
An emergency provincial lockdown that hit when Ontario kids were on their Christmas break kept them out of school until February.
"As we continue to vaccinate more Ontarians, the end is in sight, but right now these necessary measures will help to stop the spread of variants in our communities, protect capacity in our health care system, and save lives".
"If we don't think this and take action, we will be in deep trouble", the Premier continued.
Ontario's science advisers say stay-at-home orders will control the third wave of COVID-19, which is being driven by rising rates of the more deadly variants of concern.
The last time the province reported a daily case total above the 2,500 mark was on January 22, during the height of the second wave, when 2,662 infections were reported.
Brown said variants of concern are now the dominate strain of the virus in the province.
He says the province's vaccine rollout is not reaching the highest risk communities and that is delaying the impact of the vaccine.
Businesses not permitted to open include: museums, personal care services day camps, casinos and bingo halls, concert venues and more.
In preparation, the Thames Valley District School Board has asked that students take home technology/computing devices, learning materials and any required personal items such as eyeglasses and running shoes in the event of a switch to online learning during the shutdown.
Essential workers still make up the highest number of people who are being infected with COVID-19.
"I'm asking everyone to only gather with your immediate households because, my friends, I want us to have a good summer and these steps the next four weeks are our best chance to save lives and to make sure this summer we can get things open and get back to our lives". "We will get through this if we all work together".
Unifor is Canada's largest union in the private sector, representing 315,000 workers in every major area of the economy.
The province confirmed 2,557 new cases of the novel coronavirus on Thursday.