McDermott led the Royals to five straight wins at the start of last month, with two draws and a solitary defeat making it just one loss in eight as they secured a play-off place.
The 50-year-old, who was also nominated for the award in January and March, made it third time lucky by beating Norwich's Paul Lambert, Cardiff's Dave Jones and Millwall boss Kenny Jackett to the award.
Source: PA
Source: PA