Corporate profits grew massively in the first three quarters of last year, expanding by at least 25 percent in those time periods. S&P 500 earnings are expected to have grown by 12.6 percent in the fourth quarter, according to Lindsey Bell, a strategist at CFRA Research.

Shares of Amazon, Apple, Netflix and Alphabet all closed down at least 1 percent. The S&P 500 tech sector also dropped 0.9 percent.

The Dow Jones Industrial Average pulled back 86.11 points to close at 23,909.84 as Merck and Apple lagged. The fell 0.54 percent to 2,582.61, as the tech, health care and utilities sectors underperformed. The Nasdaq Composite dropped 0.9 percent to 6,905.92. Monday also marked the first time in 2019 that the major averages posted two straight losses.

Stocks fell on Monday as the U.S. corporate earnings season kicked off. Concerns over an economic slowdown in China also dampened sentiment to start off the week.

Earnings growth will be harder to come by in 2019, however, said David Kostin, chief U.S. equity strategist at Goldman Sachs. In a note to clients this weekend, Kostin said 2019 earnings growth could be as low as 3 percent as the economy slows, the dollar rises in value and oil prices remain low.

"More important for what's likely to drive the market is what companies say about what they are seeing now and what their expectations are for 2019," said Kate Warne, investment strategist at Edward Jones.

"One of the big concerns for investors has been how much earnings growth will slow. Expectations for fourth-quarter earnings have come down a lot over the last month or two. So the reassurance or worry is going to be if companies lower guidance and say 2019 looks to be a challenging year than it's already baked in."

The calendar fourth-quarter earnings season kicked off on Monday, with Citigroup reporting stronger-than-expected earnings. However, the bank also said its fixed-income trading revenue fell 21 percent.

The moves Monday come after the major indexes posted solid weekly gains last week. The Dow and S&P 500 also notched three-week winning streaks. Equities kicked off 2019 with strong gains, rebounding from a sell-off in December that briefly sent the S&P 500 into bear-market territory.

However, DataTrek Research co-founder Nicholas Colas is not sold the market is out of the woods yet.

"We are chalking up the S&P 500's YTD gain of 3.6% primarily to the cessation of tax loss selling in the New Year," Colas wrote in a note to clients. "That list of the biggest S&P 500 losers in 2018 we published last month is up an average of 9.5% YTD, a data point which supports this idea."

"From now on, US stocks will have to earn further gains. Earnings season kicks off this week, the market's first challenge. US/China trade will likely have to wait for Davos, which is 9 days away," he added.

Investors were also worried after fresh data out on Monday showed December exports and imports dropping unexpectedly in China. These figures deepened concerns of a slowdown in the world's second-largest economy.

—CNBC's Silvia Amaro contributed to this report.