The Wildcats (7-3) fell behind 8-0 in the third inning before beginning to fight their way back. UK scored three runs in the third and five in the fifth to pull even against the Bulls.
Luke Becker's double began the game-winning rally with two outs. After an intentional walk of Tristan Pompey, Arnold battled back from an 0-2 count against Buffalo closer Mike Kaelin and lined a single through the left side to score Becker for the walk-off win.
It capped a career day for Arnold, a senior catcher from Frankfort, who tied his career high with three hits. The Cats also got a two-hit, three-RBI day from Evan White and the third home run of the season by JaVon Shelby.
UK starter Kyle Cody made his 31st career start, allowing eight runs on seven hits in over two innings, walking three and striking out seven.
A strong showing from the Cats' bullpen helped them get back in the game. Logan Salow and Justin Lewis (1-0) combined to work eight shutout innings, allowing only two singles and two walks with 12 strikeouts. It marked the first career win for Lewis.
Buffalo (3-5) starter Charlie Sobieraski worked 4.2 innings, allowing seven hits and eight runs, walking five and striking out two. Kaelin (0-1) suffered the loss despite a strong relief outing, allowing the run in 3.2 innings, with three hits and one walk.
Kentucky will return to action on Tuesday, playing host to Cincinnati at 4 p.m. ET, before welcoming Murray State at 4 p.m. on Wednesday.