"Mail-in voting is important, but it is not an exclusive substitute to physical locations," Newsom said, noting that some people are simply not comfortable mailing in a ballot.
The move, resulting from an executive order Newsom signed earlier Friday, makes California the first state to make voting by mail in November an option due to the coronavirus pandemic, Secretary of State Alex Padilla said.
"This election is slated to be the most consequential election of our lifetime," Padilla added, noting that postage on ballots will be pre-paid. "We remain committed to providing as many safe, in-person opportunities to vote as possible on and before Election Day."
The move coincides with the first day of California's partial reopening and comes as concerns about in-person voting during the coronavirus outbreak have brought the longstanding political clash over voting by mail to the fore.
Newsom announced Monday that shops including clothing stores, florists and bookstores will be allowed to offer curbside pickup services starting Friday.
All reopenings in the state will be subject to active monitoring and surveillance, though Newsom noted that some counties will be allowed to move further into the reopening process if certified by health officials and county supervisors.
"We will afford them that right with conditions and modifications that meet the health needs of the entire state," he said Monday.
Newsom has proceeded with caution in reopening the Golden State, which has seen more than 62,500 cases and 2,500 deaths to date, according to data from the Johns Hopkins University Center for Systems Science and Engineering. California was among the first states to issue a stay-at-home order in March and joined in a pact with fellow Western states Washington and Oregon in April to begin incremental release of stay-at-home orders.
The vote-by-mail program marks the first time in California's history that ballots will be mailed to every registered voter in the state, according to Padilla's press secretary, Sam Mahood.
Republicans lambasted the move by the California governor, saying the election is now susceptible to voter fraud. They floated taking legal action to stop the decision.
Trump campaign Communications Director Tim Murtaugh called the move "a thinly-veiled political tactic by Gov. Newsom to undermine election security" and "a wide open opportunity for fraud."
"Everyone is concerned about the safety of voters, but jeopardizing election security is the wrong way to go about it," he said in a statement, asserting that the state "has a bad record" of maintaining its voter rolls.
Republican National Committee spokesman Steve Guest said in a statement that "we are weighing our legal options to ensure the integrity of the election."
"While we have always supported absentee voting, California is a case study in why automatically (sending) this many ballots is a problem," he said.
But multiple states are looking to such options as the coronavirus outbreak continues. Ohio held a vote-by-mail primary last month, on the heels of at least 19 people in Wisconsin testing positive for coronavirus who said they voted in-person or worked the polls for the state's primary election, according to the state health department.
Groups with backers ranging from anti-Trump conservatives as well as former first lady Michelle Obama have come out in support of voting by mail to protect voters' health and safety. Meanwhile, President Donald Trump has led the Republican charge against the effort.
Trump lied about mail-in ballots at a White House press briefing last month, claiming that "people cheat" and that "the mail ballots are corrupt, in my opinion."
Vote-by-mail advocates point out that a handful of states, including the GOP-dominated state of Utah, already conduct their elections entirely by mail, and that a number of studies have found no evidence of widespread voter fraud in the United States.