WRU Chief Executive Roger Lewis said: "Mostyn goes with the blessings and goodwill of everyone at the WRU. Those who have had the pleasure of working with him during his time at the Union have found him to be a true rugby professional.
"We would like to thank him for his evident commitment to the cause of Welsh rugby during his time as part of our elite performance team.
"The Union is undergoing a period of restructuring in its rugby department and recent announcements about a new Head of Rugby Development at executive level and four new positions below that - to look after the specific areas of national performance, rugby, coaching and referee development - will ensure that the area of our business Mostyn is leaving remains in good shape to build for the future."
During his time at the Union Richards was part of a team responsible for many achievements including the establishment of the four successful WRU Regional Academies, the restructuring of age grade rugby and the re-establishment of the Wales Sevens International Squad.
"The past 17 years have been very challenging, but extremely rewarding for me both personally and professionally," said Richards.
"I have been fortunate enough to progress from the 'shop floor' of the WRU to work at senior executive and Board level. I have seen many changes at the Union and worked with many professional and hard working people.
"Welsh rugby has a bright future and I feel privileged that I may have played a part in shaping the way forward.
"I feel it's now time to look for a new challenge and fresh horizons and I will endeavour to take with me the many years of experience and knowledge gained at the WRU into my next professional position."