"We would like to thank John and Martin for all their efforts since their appointments and wish them well for the future," a club spokesperson said.
"There is much to do but for now the focus must solely be on the remaining games of this season," said Jenkins, whose previous stint as CEO lasted 14 years.
West Brom are bottom of the league after 27 matches, with 20 points after three wins, 11 draws and 13 defeats. They have lost their last three league games.
The Midlands club made a bright start to the current season with victories in their first two games before going 20 games without a win. They sacked manager Tony Pulis and replaced him with Alan Pardew in November.
Williams was appointed chairman of West Brom in 2016 when Chinese businessman Guochuan Lai, controlling shareholder of Yunyi Guokai (Shanghai) Sports Development Ltd, completed his takeover of the club.
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by Christian Radnedge)
