He played in Major League Baseball (MLB) as a third baseman from 1973 to 1987 for the Baltimore Orioles, California Angels and St. Louis Cardinals.

[1] When the Orioles' Hall of Fame third baseman, Brooks Robinson, retired at the end of the 1977 season, DeCinces was given the difficult task of replacing the legendary player.

[3][5] On June 22, 1979, DeCinces hit a game-winning home run at Memorial Stadium off Detroit Tigers reliever Dave Tobik.

Eddie Murray reached base on a single, and, with two outs, DeCinces hit a two-run home run to give the Orioles a 6-5 victory.

DeCinces tagged out Dan Ford who was attempting to advance to third base on a force play that ended Game 2 of the 1979 American League Championship Series.

[13] The deal was delayed when Ford requested additional compensation because the Orioles were not one of six teams listed in his contract to which he could be traded without approval.

[1] Released by the Angels on September 23, 1987, he concluded his major league career by playing in four games for the St. Louis Cardinals late in the 1987 season.

On August 4, 2011, DeCinces, along with three others, was charged by the Securities and Exchange Commission (SEC) with insider trading ahead of a company buyout.

[17] In November 2012, DeCinces received a criminal indictment on insider trading related to the same incident and was charged with securities fraud and money laundering.

[18] On May 12, 2017, after a nearly two-month trial, a federal court jury in Santa Ana, California, found him guilty on 13 felony counts.

[22] Former Angels teammate Rod Carew extolled DeCinces' charitable contributions at the sentencing, telling the court, "I am here because he has done so much more for other people".