After leaving the US Air Force in 1976 Mostyn earned a B.S.E.E. degree from the University of California Santa Barbara. He practiced engineering at Raytheon in Santa Barbara, developing infrared focal planes, but quickly learned that his best skills were in people and project management and spent the bulk of his career managing engineers.
While still at Raytheon, Mostyn began preparations for his next career. He took many skill based clock restoration courses and began a clock repair business. He also earned a Certificate in Appraisal Studies from University of California Irvine in 2011.
Mostyn retired from Raytheon in 2012 to spend his time in horological pursuits. He has been the Courthouse Clock Master for the Santa Barbara County since 2008 and completed a full restoration of the 1929 Seth Thomas tower (turret) clock in 2012. This restoration, including the renovations made to the clock room itself, now known as the Bisno Schall Clock Gallery, has received multiple awards: Winner of NAWCC Annual Craft Competition, 2012; Winner of Santa Barbara Beautiful Award for Historic Revitalization, 2013; and recipient of the California Governor’s Award for Historic Preservation, 2015.
In 2015, he and wife Debbie traveled to England where he earned a Master of Arts degree in Conservation Studies and a Diploma in Restoration of Antique Clocks at West Dean College. Whilst at West Dean, he was awarded the BADA Geoffrey Moss Prize for best contribution to the clock programme.
He is the current president of Chapter 190 of the National Association of Watch and Clock Collectors and a member of:
- American Institute for Conservation of Historic and Artistic Works (AIC)
- Institute of Conservation (ICON, UK)
- International Institute for Conservation of Historic and Artistic Works (IIC)
- National Trust for Historic Preservation (USA)
- Antiquarian Horological Society, UK, and USA Section
- Freeman of the Worshipful Company of Clockmakers, London
- British Horological Institute
- American Watchmakers-Clockmakers Institute
- American Society of Appraisers, Connoisseur
- American Alliance of Museums
He now operates his own conservation studio in Santa Barbara;