The 38-year-old has managed the Pilgrims' second string for the past two seasons and guided United to first and third place finishes respectively.
He has now been given the opportunity to take over the managerial reigns at Eslaforde Park following the retirement of long-serving Brian Rowland.
Maddison's replacement at The Jakemans Stadium will be announced in due course.
Everybody at Boston United would like to wish Maddison well in his new role and thank him for his sterling service during the last two seasons.