United's Jamie Murphythreatened a couple of times early on, putting one effort wide and having a free-kick saved by Bartosz Bialkowski.
After County's Danny Haynes had a header well saved by George Long, the visitors were reduced to 10 men moments later when Gary Liddle was sent-off in the 14th minute following a reckless challenge on McDonald.
McDonald put United in front 16 minutes later with a 12-yard shot after meeting Darryl Westlake's low cross.
County's Joss Labadie headed wide after finding himself unmarked and United's Chris Porter then put an effort off-target from a great position before County equalised.
Substitute Showunmi, who had only been on the pitch for a few minutes, fired past Long in the 59th minute after a long ball forward dropped to him just outside the area.
Maguire scored the winner eight minutes later with a far post header from Stephen McGlinn's right-wing delivery and the Blades then missed a host of late chances to extend their lead.
- Sports & Recreation
- Enoch Showunmi
- Harry Maguire