Arnold W. Jacobsen was born in Dilly, Oregon, on July 5, 1913, the son of Anton O. and Alida G. Svendensen Jacobsen. In 1915 the Jacobsen family moved to Butte, Montana. In 1926 Arnold left school to work on ranches in the Jefferson Valley. He returned to school in 1928 and graduated from the eighth grade. He held a number of jobs in Butte and elsewhere from 1928 until 1935, when, in partnership with his fiancee Lois Shea, he opened the Baby Bunting Shop in Butte. By 1947 the couple had seven stores in three states and employed thirty staff, including managers who were partners in their respective stores. After divesting themselves of their stores, the couple bought a seven-unit cabin camp on Whitefish Lake in February 1951, and opened the Win-a-Kee resort. By 1960 the Jacobsens had improved the Win-a-Kee to a sixteen-unit resort. The Jacobsens sold the resort in 1972.

Arnold served as president of the Whitefish Chamber of Commerce and helped organize the six-member Associated Chamber of Commerce, and served as the latter's second president. In 1959 he helped to organize the Chamber of Commerce's Whitefish Winter Carnival and was honored as the Carnival's King Ullr XII in 1972. In 1970 he worked for the Montana Chamber of Commerce as a field representative. He also served on the Montana Chamber's Board of Directors and the Montana Travel Hosts Steering Committee during the early 1970s. Jacobsen and others initiated the Whitefish Lake Stabilization Project in 1968. The City of Whitefish elected to pay taxes to the U.S. Soil Conservation Service in order to qualify for their services. Jacobsen served on the Soil Conservation Board representing Whitefish from 1969 to 1981.

In 1971 Arnold Jacobsen successfully ran for election as a delegate to the 1972 Montana Constitutional Convention where he served on the Local Government Committee. While serving as a delegate, Jacobsen championed private enterprise issues, such as the privatization of Montana's state liquor stores, as well as other causes, such as a constitutional ban on firearm restrictions. Jacobsen served on the Flathead County Study Commission in 1972. The Commission, one of 182 city, town, and county local government study commissions, was mandated by the 1972 Montana Constitution and was established to review the existing governmental structure and to submit one alternative form of government to the electorate by 1976.

The Flathead County Park Board (later retitled the Flathead County Parks and Recreation Board) was organized in March 1974. Arnold Jacobsen became an active member of the Board and served as chairman in the mid 1970s. Board President Owen Sowerine urged the creation of a "natural area" on a 442-acre island at the confluence of the Flathead and Stillwater rivers. Owen Sowerine died in January 1975, but the project continued, and the formal dedication of the Owen Sowerine Natural Area was held on September 9, 1978. In 1975 Arnold and Lois Jacobsen began to organize the Soroptimist Children's Park. The Park was completed in 1977. Lois Shea Jacobsen died in April 1981, and Arnold married Lucille Barstad Smith in 1982.