The pair have been given notice of termination of their contracts and been placed on gardening leave with immediate effect, the club announced on Tuesday.
Mark Jenkins, a director of West Brom's controlling company WBA Holdings, returns to the club as CEO having previously held the position for 14 years, and he highlighted the importance of results in the Premier League following his appointment.
The first club chairman in the Premier League to be sacked due to poor results has admitted his surprise at getting the chop.More news: Five Die as Stolen Car and Oil Truck Crash on Long Island
"There is much to do but for now the focus must exclusively be on the remaining games of this season", said Jenkins, whose previous stint as CEO lasted 14 years.
Pardew's appointment in November failed to trigger a "manager bounce" with his current record standing at three wins, five draws and seven defeats from 15 league and cup matches.
The Baggies added that they will be making no further comment.More news: USA healthcare spending to rise 5.3% in 2018
Williams was named chairman in 2016, shortly after the club was purchased by Chinese billionaire Guochuan Lai.
But the expected recovery under Pardew hasn't materialised.
The bottom club in the Premier League, now seven points from safety with 11 games of the season to go, have won just one of their last 25 games in the league.More news: Vladimir Putin meets Federation Internationale de Football Association president Gianni Infantino to discuss World Cup
Pardew has won one game less than Pulis did - collecting two fewer points and his win rate of 7% is less than half of Pulis's 15% during his worst spell at The Hawthorns.