The Pelicans are also receiving several draft picks in the the deal, according to Tim Bontemps, but none are coming from the Pistons.
Mirotic, 27, is averaging career-highs in both points (16.7) and rebounds (8.3) per game in 2018-19 while shooting 44.7 percent from the field and 36.8 percent from three in 32 games (22 starts).
Smith has appeared in just 18 games this season, 12 of them with the Washington Wizards and six of them with the Bucks.
The 21-year-old Sudanese player was selected by Milwaukee by the 10th overall pick in the 2016 draft. Mirotic was the No. 23 overall pick in the 2011 NBA Draft.
Both Harris and star Sixers guard Jimmy Butler, acquired from Minnesota earlier this season, are free agents this off-season while Simmons is in line for a contract extension in the near future. His play time declined on this season and what was supposed to be a building piece, was now at the bottom of the bench for the Bucks.
"Tobias is one of the best three-point shooters in the National Basketball Association and possesses an innate ability to impact the game on both ends of the floor, while Boban and Mike provide valuable skillsets, size and depth to our team".
Smith was acquired by the Bucks from the Wizards in a three-team trade on December 7, 2018.