This work reports for the first time an exceptionally high thermal stability of cellulosic nanofibers films at 190 °C after a 5 h exposure, a threshold required for their design and manufacturing in flexible electronic devices. A long-term durability validation of a flexible cellulose nanofiber over a period of 10 years exposure under ambient condition validated their life-span over a long period of operating condition. Arresting quinoid oxidative reactions pathways and thermal degradation of lignin or lignin-less cellulose are demonstrated to be two fundamental routes to enhance thermal processing of cellulose nanofiber substrates and to achieve long service life of energy devices made from them. One of the main highlights of this work is the first time validation of a long operating service life for a light-emitting device made from a cellulose nanofibers substrate, while continuously illuminating the fabricated prototype under ambient condition for an equivalent of 10 years. The unique durability of this flexible substrate will have potential applications in flexible lighting, energy and sensing devices. © 2020 The Authors