Bucks finished runners-up in this division last season and made a decent start after winning the toss and batting first on Sunday, with Shelvin Gumbs (71) and captain Michael Payne (65) guiding them to 137-2.
But once those two departed, the rest of Bucks’ batting line-up struggled to stick around with Patrick Castleden (22) the only other man to pass 20 in the team’s 239 all out.
Northumberland appeared on course for a formidable total when they reached 178-2 in reply but Saif Zaib (3-52) kept the visitors in check, although a quickfire unbeaten 71 from Jarvis Clay allowed Northumberland to declare on 350-7, a lead of 111.
Bucks again made a strong start when they came to bat for a second time as Gumbs (67), Zaib (43) and Payne (30) steered them up to 110-1, but once again only Castleden (59) could make a telling contribution in their 263 all out.
That left Northumberland needing 153 to win and they encountered few problems in chasing down the target despite two wickets for Payne, winning by six wickets.