He was one of five Chester managers during the 2001/02 season, taking over from Gordon Hill in early October.
But, after starting with two victories, City were to win just one of their next 10, and he departed following the 3-1 Boxing Day defeat at Northwich.
His last management post, as assistant to Dave Pace at non-League Droylsden, ended just before Christmas.
Mungall's return to the Deva is the second addition to Williamson's backroom team this week following the appointment of ex-Liverpool, Blackburn, Bolton and Tranmere midfielder Jason McAteer to the coaching staff.
That is Chester's response to a sorry return of just two points out of a possible 33 since Christmas Day.
This nightmare 11-game run, in which they have shipped 30 goals, has seen them fall away from being likely promotion challengers to even be considered outside relegation candidates.