Mountford, who joined the retailer's board as trading director in 2002, will step down from the board on Monday and leave the retailer on January 31.
Moss Bros non-executive director Brian Brick, who used to head Moss Bros rival SRG, will be appointed chief executive on an interim basis.
Bernie Myers will step down as non-executive director on Monday. The board is seeking new independent non-executive directors.
Chairman David Adams said:
"l am delighted to welcome Brian to the role of interim chief executive. Brian has a wealth of experience in the menswear retail sector and l look forward to working with him in his new role as interim chief executive whilst candidates for the permanent office of chief executive are considered. "