Athlone should have taken the lead after just three minutes but Brian Gill’s poor effort was well smothered by Eric Browne.
However, six minutes later the excellent Sherlock headed Gill’s cross past Liam Grant, via the underside of the crossbar.
SD Galway were not without their chances as Enda Curran and Mike Tierney both came within inches of snatching a point for the home side.
Substitute Tommy Barrett could have made it two for the visitors with a minute remaining when he robbed Brian Geraghty of possession, but the SD Galway skipper recovered well as Michael Kerley’s side held on for a deserved win.
SD GALWAY: Grant; Harty, Browne, Geraghty, Bradshaw (Fadden, h-t); Conlon (Lyons, 78), Greaney, McConnell (Dusi, 65), Gaffney; Tierney, Curran.
ATHLONE TOWN: Jennings; Scullion, Danaher, McCarthy, Kenny (Relihan, 72); Walsh (Hughes, h-t), Coleman; Broughall, Gill, Williamson; Sherlock (Barrett, 58).
Referee: Sean Grant (Wexford).