Dennis Hendrickson was born in 1930 in Ypsilanti Township, Stutsman County, North Dakota. His parents were Ralph and Grace Hendrickson. He grew up on the farm that he later bought from his father, receiving most of his education in Ypsilanti Township, and graduating from Jamestown High School in 1950.
He married Dorothy Aljoe, the daughter of John and Marie Aljoe, in 1952. Dorothy had attended grade school in Corwin Township, then two years of high school in Montpelier, and graduating from Jamestown High School in 1950.
After their marriage, Dennis and Dorothy lived on the Hendrickson farm, which had been homesteaded by his grandfather, Chris Hendrickson, and which Dennis later bought from his father in 1970. Dennis built his house on the west side of the river in 1954, across from his father's home.
Dennis and Dorothy had one son and two daughters: Dwight, Dorinda, and Dianne, all of whom attended school in Montpelier.