A terraformer, Doctor Lon was one of the many Federation scientists who worked on the project to help the planet Mestiko recover from the disastrous aftereffects of "The Pulse" that damaged their ecosphere in 2265. To that end, Lon designed a satellite network which would convert dangerous nitrogen oxides in the atmosphere to atmospheric oxygen and nitrogen.
When Dr. Leonard McCoy visited with Lon circa 2272, he described him as having mellowed, though he felt he had failed to form a personal connection with the native Payav. (TOS eBook: Mere Anarchy: Shadows of the Indignant)
Following the mar-Atyya's revolution in 2274, when all "Dinpayav" were expelled from the planet, Lon resolved to remain behind, undergoing surgical alteration to appear as a Payav. However, his lack of identification with the Payav gave him away as an imposter, and was nearly killed by a mob before being saved by Daki orGalya. With her guidance, Lon began to make himself into a native of Mestiko, eventually taking the name Cart etDeja. He also married Daki, and had two children with her. (TOS eBook: Mere Anarchy: The Darkness Drops Again)