The game was very much in the balance until centre Bevan Duffy's late intercept try delivered Boyne their second Towns Cup in-a-row.
Offaly side Tullamore had the the better of the opening exchanges and powered into a 10-0 lead with full-back Niall Geraghty kicking a penalty and converting a try from hooker Cathal Feighery.
Boyne bounced back with a pushover try from prop Paddy Cooney, which influential full-back Niall Kerbey failed to convert.
Both Geraghty and Kerbey were wide with kickable penalties, and though the latter split the posts with a 22nd minute effort, Tullamore were soon over for an unconverted, scored by centre Ivor Scully.
Boyne enjoyed a purple patch coming up to half-time, with a converted try from replacement Adam Sherrard levelling this high quality game at 15-15.
That score came while Tullamore's Aaron Deverell was in the sin-bin, but the out-half redeemed himself four minutes before the break when he finished off a fine try, with Geraghty's missed conversion leaving the final tantalisingly poised at 20-15.
Crucially, Boyne came out for the second half a renewed force. Kerbey and scrum half and captain Craig McGrath both crossed for tries in a seven-minute spell as the holders moved ahead.
Boyne missed chances to move further ahead when two penalties went a-begging, but their defence was simply magnificent when Tullamore tried valiantly to go in search of further tries.
The result was evidently sealed six minutes from the finish as Duffy swooped on a loose pass to dart over from 50 metres out. Kerbey's conversion from in front of the posts made it 32-20.
Tullamore gained some consolation when hooker Feighery crossed for his second try in injury-time, after a bout of forward pressure - but Boyne ran out deserved winners.