The 50-year-old, who spent his entire playing career with Grimsby Town, has been confirmed as Craig Elliott's new right-hand man for the 2019-2020 campaign.
McDermott has considerable experience in National League North, having spent a successful spell as Simon Weaver's assistant at Harrogate Town before forging his own managerial path at Alfreton Town.
Elliott told bufc.co.uk: "I have been searching for an experienced assistant for the past two or three months and John certainly fits the bill.
"He has vast knowledge of the non-league scene and is an excellent coach.
"He has plenty of enthusiasm to be a real success here at Boston United."
Lee Stratford will assume the role of first team coach, while Rich Lawrence and Richard Boryszczuk will retain their respective goalkeeping coach and chief scout duties.
PIC: Mansfield Chad.