Townsend, 36, spent seven years at United over which time he held several positions, including HR manager, heading up the museum and tour centre and overseeing other special projects such as the 2003 UEFA Champions League final that was staged at Old Trafford.
Townsend was also head of HR at Everton and over four years helped restructure the club, including the outsourcing of the retail and catering divisions.
A year ago he was made chief executive of the Everton Foundation, one of the country's leading football charities.
On his appointment at Meadow Lane he said: "I am delighted to join Notts County and I thank (executive chairman) Peter Trembling for the opportunity to be part of the challenge of making County great again.
"My previous positions have involved a great of work deal within the community and experience of growing fan bases, and this is something that I feel will be particularly applicable to the project at Meadow Lane."