The 34-year-old left Tottenham Hotspur’s coaching staff to take charge of Albion on a three-year deal, but has been unable to turn them into promotion contenders.
Despite the early promise of three wins in their first four league games this season, the momentum did not last, with only six more victories in the 22 since then.
A fourth defeat in five games either side of Christmas has now cost Mason his job and left the club looking for a third permanent boss in the past 12 months.
Speaking following the game at Leicester, which the Baggies lost after an added-time goal by the home side, Mason told BBC WM: “We are on a terrible run in terms of away games, but you look at the last seven or eight that we have played, we have had more shots at goal than the opposition, more shots on target, more chances created, and conceded so little at our end as well.
“I am proud of the team and proud of the performance, and we deserve so much more.”