Wall Street closed out a dismal, turbulent year for stocks on a bright note Monday, but still finished 2018 with the worst showing in a decade.

After setting a series of records through the late summer and early fall, major U.S. indexes fell sharply after early October, leaving them all in the red for the year.

The S&P 500 index, the market's main benchmark, finished the year with a loss of 6.2 percent. The last time the index fell for the year was in 2008 during the financial crisis. The S&P 500 posted tiny losses in 2011 and 2015, but eked out small gains in both years once dividends were in...