Sources told ESPN and The Athletic that the deal was agreed to in principle on Wednesday. Johnson, 22, was the No. 8 overall pick in the 2015 NBA Draft. Maker, 21, was the No. 10 overall pick in the 2016 NBA Draft.
Johnson was averaging 7.5 points, 3.6 rebounds, 1.3 assists and 1.0 steals per game this season for Detroit. Maker was posting 4.7 points and 2.7 rebounds per game this season for Milwaukee.
Bucks star Giannis Antetokounmpo commented on the swap following Milwaukee's 148-129 win against the Washington Wizards on Wednesday in Milwaukee.
"Thon is like my little brother, this one hurts more than any trade that happened that I've been a part of but this is the business we work in," Antetokounmpo told reporters.
Antetokounmpo also called Maker "the heart and soul" of the Bucks.