Frithjof was born on October 25, 1898 in St. Charles, Kane County, Illinois. He was the third child of Joseph and Emma Rosenquist. He was under two years of age when the family moved to Clay County, Minnesota, where Frithjof grew up and attended school. He left home occasionally to do seasonal farm work with relatives in Illinois. In 1927, he worked briefly with his brothers, Archie and Harold, on the Moffat tunnel project in Colorado. For several years, he worked at Kost Brothers gravel company at Glyndon.  He moved to a farm in Minerva township, Clearwater County, in the 1940's.

There were many stories told about the adventures of the Rosenquist boys while they grew up in Minnesota. Along with the hard work and responsibilities, there were many opportunities to enjoy life in the country. A few of those stories still survive. Frithjof's brother, Archie, recorded some of them in 1967. He told about the well-known school shooting incident that took place in March of 1913. When Archie told about the day a man came to school and shot their teacher, he didn't mention Frithjof, but he was there that day. It is said that while the other students were frantically running to escape the gunman, Frithjof was calmly reading a book, so focused that he was oblivious to what was going on around him. He was the only student who took time to put his coat on before going home.

Archie also remembered this incident involving Frithjof : "Every fall the crows came by the thousands in the evening and roosted in some trees about a half mile from our place. Frithjof and I sneaked up under them one moonlit night and shot into the flock with shotguns. We had crows falling all over the place. We did that about three nights in a row and then they changed their roosting place."

Farm work didn't always go as planned. Archie told this about himself and his older brother: "Frithjof and I were plowing one day and had stopped for lunch. We fed the horses oats and let them loose to eat grass. A big hail storm came up. We crawled under the wagon but the hail stones were so big that the horses all started running with the storm. It took us most of the afternoon to round them up."

Frithjof had a car when it was rare to own one. Archie told this: "Frithjof bought a second-hand car from my Uncle Herbert, an old time car with no top, a touring car. It had presto lights for headlights and he had to light them with a match. We went to Glyndon one Sunday and the car stalled about a mile outside of town. We couldn't get it started and had to walk home - about eight miles."

Frithjof went into the service in November of 1942. He began his training in gunnery, but was later assigned to a searchlight battery, part of an anti-aircraft battalion. He trained in Texas and Florida before being sent overseas on Dec 4, 1943. He served as a replacement cook for a time during his overseas duty. While he was stationed in New Guinea, he developed a fungus disorder on his feet due to the dampness. He was returned to the States on November 14, 1944.

Frithjof was hospitalized in Clinton, Iowa, and Camp Carson, Colorado. He had sores on his eyes as well as his feet. The sores healed, but Frithjof's hospital stay was extended because of dental problems. Several of his upper teeth had to be pulled and he had to receive treatment for an infection that had developed near a wisdom tooth. Frithjof expected to be reassigned or sent on furlough after his recovery, but he was discharged on October 5, 1945.

Frithjof lived on his farm in Minerva Township after the war. He built a garage with an upstairs apartment and lived there until the late 60's. He worked his farm, cut timber in the winter, and occasionally did temporary work for contractors in North Dakota.

In 1969, Frithjof suffered several light heart attacks. He was hospitalized for a time and was forced to become less active. He stayed at Peggy's place for a while and later boarded with Bill and Elna. In the summer of 1971, he began traveling, visiting his brothers' families and seeing the country. He spent the winter of 1971-2 in Tucson, Arizona. The next winter, he stayed in Florida, after visiting Archie and Esther in New York and Althea in North Carolina.

In the fall of 1973, Frithjof moved into a small apartment in Bagley after living briefly at a small house just south of town. He remained fairly active until early 1976, when he had to give up driving because of failing eyesight. Later that same year, he took another extended trip, visiting relatives around the country. The next year, he left his apartment and entered Greensview Nursing Home in Bagley. Frithjof Theodore Rosenquist died in Bagley on July 21, 1985.

Frithjof is fondly remembered as an intelligent man and an avid reader. He had an opinion on almost any subject and was not hesitant to share his thoughts. He was a very kind and considerate person. Ruth remembers when she began having problems with her eyesight. It was Frithjof who purchased her first pair of glasses. He was willing and able to help his brothers and sisters when they needed guidance or help. With no family of his own, Frithjof took an interest in his nephews and nieces. He loved his mother dearly and was there to comfort her in her later years.

Frithjof was a member of Redeemer Lutheran Church and the Bagley American Legion. He enjoyed playing cards, gardening, reading, and making rugs out of plastic bags.

Lord Jesus, I call upon you in my time of trouble. I am trusting in You to bring me into Your presence. Give me the peace I need.