The Phillies' record under Sandberg, who debuted as a Major League manager in August 2013, was 119-159. They are 26-48 this season and in last place, 14 1/2 games out in the National League East.
"It's a difficult day, a challenging day and a tough day for myself," Sandberg said at a news conference at Citizens Bank Park in Philadelphia. "It was not an easy decision. In a lot of ways I'm old-school and I'm very, very much dissatisfied with the record and am not pleased at all by that."
Mackanin, 63, has 106 games of experience as a Major League manager, both times as an interim skipper, first for the 2005 Pirates and then for the 2007 Reds.