Federal investigators have secured a warrant to examine newly discovered emails, a source familiar with the matter said on Sunday.
Clinton had opened a recent lead over her unpredictable Republican rival Donald Trump in national polls, but it had been narrowing even before the email controversy resurfaced, according to Reuters.
The Mexican peso, which strengthens along with the chances for a Clinton win, fell, while the U.S. dollar remained weak against other major currencies.
"There seems little doubt that a Trump victory would trigger selling in stock markets from current levels," Rick Spooner, chief market analyst at CMC Markets in Sydney, wrote in a note.
"This has traders nervous as they start the week assimilating fresh news on Hillary Clinton’s email problems."
MSCI's broadest index of Asia-Pacific shares outside Japan hit a six-week low on Monday before recovering up 0.2 percent. It is set to end the month down 1.6 percent.
Japan's Nikkei, which touched a six-month high on Friday, slipped 0.4 percent on Monday, but remains poised for a monthly increase of 5.5 percent.
China's Shanghai Composite index fell 0.4 percent, paring gains this month to 2.9 percent. Hong Kong's Hang Seng slid to a two-month low but revived to gain 0.3 percent, on track for a 1.1 percent loss in October.
On Friday, Wall Street and the dollar closed lower, after Federal Bureau of Investigation Director James Comey sent a letter to the U.S. Congress informing it that the agency is again reviewing emails related to the private server Clinton used when she was secretary of state.
Markets have tended to see Clinton as the candidate of the status quo, while there is greater uncertainty over what avictory for Trump might mean for U.S. foreign policy, international trade deals and the domestic economy.
Comey had decided in July that the FBI was not going to seek prosecution of Clinton for her handling of classified materials.
Trump had fallen in the polls for most of the past six weeks, but an ABC News/Washington Post poll released on Sunday showed Clinton with a statistically insignificant 1-point national lead.
The dollar hit a three-week high against the Mexican peso on Friday before closing up 0.7 percent. It retained most of those gains to trade at 18.9492 peso on Monday.
A Trump victory has been viewed as a key risk for the Mexican currency given his promises to clamp down on immigration and redraw trade relations with the country.
The dollar gained 0.1 percent to 104.885 yen on Monday, and remains up 3.5 percent for the month. On Friday, it hit a three-month peak after third-quarter U.S. economic growth beat expectations, before ending the day down 0.4 percent on the news about the Clinton email investigation.
The euro retreated 0.25 percent to $1.0964 after jumping 0.9 percent on Friday. It is poised to end October with a 2.5 percent loss.
Adding to the list of potential market-moving events this week is a raft of factory PMI data on Tuesday for many economies; central bank policy meetings, including Japan and Australia on Tuesday, the U.S. Federal Reserve on Wednesday and the Bank of England on Thursday; as well as U.S. October non-farm payrolls on Friday.
Oil prices extended their slide - driven by renewed oversupply concerns - and have surrendered most of the gains made in the first half of October. They are set to end the month with meager gains.
The latest oil woes were prompted after non-OPEC producers failed to make any specific commitment to join the Organization of Petroleum Exporting Countries in limiting output to support prices on Saturday.
U.S. crude slid 0.4 percent to $48.51 a barrel on Monday, up 0.6 percent for the month, while global benchmark Brent also retreated 0.4 percent to $49.47, up 0.8 percent in October.