Major Jellis Fonda was the most prominent of the early Fondas. He was the first Mohawk Valley merchant west of Schenectady in the period immediately before the Revolution. He was a close friend and associate of Sir William Johnson and accompanied him on his military expeditions as commissary. He served in the French and Indian War under Sir William Johnson in command of Scouts (lieutenant in 1755 and captain in 1756) and was commissioned Major in 176? spending much of his time during this period in Indian country.
Jellis was elected as one of the four assistant judges of Tryon Court in 1773; became State Senator, Western District, 1777-81 and 1788-91; Fulton County Judge 22-Mar-1784 and Commissioner of Indian Affairs. He was one of the executors of the will of Sir William Johnson and designated as guardian of his children. Jellis Fonda is frequently referred to in the novel "In the Valley" by Harold Frederic and also in the novels by Robert W. Chambers dealing with Mohawk Valley and Tryon County history.

Died, universally regretted, at Caughnawaga, in Montgomery County, on Thursday last, aged 64, the Hon. Jelles Fonda, Esq. and on Saturday his remains interred in the Family Burrying Ground by a numberous and respectable consourse of citizens from Montgomery, Albany and Schenectady. In public life he supported the character of firm patriot and friend to the true interest of country; and in private, he was universally esteemed a valuable member of society, and displayed those amiable qualities which endeavor unto man. He has left to bemoan his legacy a numerous train of dependant strangers, a widow and respectable offspring, as well as relatives and intimates. Albany Register (Albany, NY), Monday, June 27, 1791.