The former scrum-half (pictured right) has also represented Halifax and Doncaster as a player and joins as an assistant to head coach Lee Greenwood.
Nathan Graham was recently confirmed as the newest member of the coaching staff for next season following the departure of Jack Howieson.
Mennell already has experience of coaching at the Tetley’s Stadium, having coached the club’s reserve side in 2015.
“Paul is a great addition to the coaching staff,” said Greenwood.
“He was a very smart player at this level and has good ideas about the game.
“He enjoyed his time at Dewsbury before, coaching the reserve team and he’s happy to be back involved.
“You can never have enough good people involved in your club so Paul will fit in nicely alongside Nathan Graham, Paul Sykes, Adam Mitchell and myself.”
Greenwood also welcomed the addition of Super League player Reiss Butterworth to his playing squad for 2021.
The hooker has previously represented Batley and joins the Rams looking for more first-team action.
Greenwood added: “We’re delighted to bring Reiss to Dewsbury Rams. We were looking for a quality nine to come in and when we knew of his availability we moved quickly.
“He ticks a lot of boxes for us - young, hungry with loads of potential - but has already shown he can play at Championship level.
“There are elements to his game we can help him with but all the raw materials are there.”