Hammond said the club decided not to offer Boylan a contract after he stepped in to replace the fired Scott Skiles in January and guided Milwaukee to a 22-28 mark.
The eighth-seeded Bucks lost in four-game sweep to the Miami Heat in the first round of the Eastern Conference playoffs.
Boylan had served as the team's top assistant coach for four seasons before being tapped to replace Skiles.
"At this time we feel it's in the best interest of the organization to seek a new coach to lead our team," Hammond said. "We appreciate Jim's efforts not only in his time as head coach, but in his entire tenure as a coach in Milwaukee."
The team has launched a search for a new head coach, the GM said.
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