Nine players arrived at Albion over the course of the transfer window - with two coming in on deadline day in the form of Samuel Iling-Junior and Charlie Taylor.
Mason now knows what his squad will look like until January - and what he will have at his disposal.
Mason's sides so far this season have been pretty settled barring one of two changes. But what XI represents his best side available after the transfer window has slammed shut?
Lewis Cox and Jonny Drury give you their take on what West Brom's best XI looks like:
Lewis Cox
Griffiths, Gilchrist, Phillips, Mepham, Styles, Mowatt, Collyer, Johnston, Iling-Junior, Price, Maja