Dennis was born in Hancock, Michigan, USA, on 10 September 1909 to Frederick and Anna (née Erickson) Carlson. Her parents were Swedish immigrants. She had two older sisters, Jennie and Emma, and one younger sister, Agnes. Her parents were charter members of the Swedish Lutheran Church (Salem Lutheran Church) which was one of the congregations that formed Gloria Dei Lutheran Church in 1970.

In 1930, she married Paul Dennis and they had a son, Lowell (1938-2011).

Dennis worked as an accountant until she was 70, but kept a few clients up until she turned 90. She cleared snow until the age of 100. At the age of 105, she moved out of her home into a long-term care facility in Michigan, USA. For her 108th birthday in 2017, the Keweenaw Brewing Company created a special beer called “Julia 108”, and Dennis was involved in the project.

In her later life, Dennis lived in a nursing home in Hancock, Michigan. She died on 20 February 2022 at the age of 112 years, 163 days. She was survived by three granddaughters, seven great-grandchildren and one great-great-granddaughter.