Maynard, whose cricketer son Tom died in tragic circumstances last year, will leave the Titans after two years in charge.
Under Welshman Maynard, the Titans won the four-day and Twenty20 domestic titles last season, and they will face Highveld Lions in the T20 final on Sunday, the final match of the 2012/13 campaign.
Maynard, who also worked as an assistant coach with the England team from 2004 to 2007, will leave the franchise after the match.
He said in a statement: "I am most appreciative of the opportunity I have been given at the Nashua Titans for the past two seasons and am very proud of my contributions to the team's success that we have enjoyed.
"Winning the SuperSport Series and the coveted T20 Challenge in my first season here and now leading the team to another final on Sunday is a testament to that. I leave for personal reasons."