Her new appointment was confirmed by the league at the weekend when it was announced that executive officer Duncan Bayley would step down after almost 30 years in the league's administration.
Firth's appointment comes as part of an 18-month review of the league's administration systems.
She will be one of three league officers, headed by league secretary Phil Bradley and completed by operations manager Alan Allcock.
Bayley, who joined the league as secretary in 1979, will continue to act as a consultant to the competition.
League chairman Peter Maude said: "Duncan has been a tremendous servant to the competition and has been largely responsible for the excellent reputation we enjoy today.
"He has also managed Unibond's sponsorship of the league superbly well, and it is through his efforts that we have been able to maintain the relationship with Unibond over a record-breaking 15 seasons.
"He will be a hard act to follow but in carrying out the administration review the board were determined to create a clear succession plan, and the appointment of Angie Firth is a key step to securing the long term future of the Northern Premier League."
Fixtures for the 2009-10 campaign will be released on July 15.
Hull boss Phil Brown has confirmed he is interested in bringing Michael Owen to the KC Stadium.
The former England striker is out of contract at Newcastle at the end of the month and is looking for a move back to the Barclays Premier League.
Fixtures
European Under-21 Championship (7.45): Finland v Spain, Germany v England