WASHINGTON—The U.S. economy slowed to a crawl at the start of the year as businesses slashed investment, exports tumbled and consumers showed signs of caution, marking a return to the uneven growth that has been a hallmark of the nearly six-year economic expansion.

Gross domestic product, the broadest measure of goods and services produced across the economy, expanded at a 0.2% seasonally adjusted annual rate in the first quarter, the Commerce Department said Wednesday. The economy advanced at a 2.2% pace in the fourth quarter...