Allen Sheeley Janssen was born in Boise, Idaho, September 12, 1908, the son of Alfred E. and Edna M. (Sheeley) Janssen. After graduating from Boise High School he attended the University of Idaho, earning a B (Arch) in 1930, a B.S. in Civil Engineering in 1933, and his M.S. in Civil Engineering in 1937.

Following his graduation in 1930, he worked for the Idaho Department of Highways. In 1931 he returned to the university as an instructor in the Civil Engineering Department. He continued teaching at the university for over forty years, becoming dean of the College of Engineering in 1946. He resigned as dean in 1967, but continued to teach half-time until his retirement in 1972. While at the university he served on the Academic Council, Research Council, Research Foundation, Patent Committee and Water Resources Committee; he was also faculty advisor to the Delta Tau Delta Fraternity from 1936 to 1956.

While teaching he also acted as consultant and designer on many civil engineering projects, with emphasis on highway work, materials of construction, and construction and analysis research. He also worked as a field draftsman for the Idaho Department of Highways from 1930 to 1946. After his semi-retirement from the university he worked part-time as an office engineer for Pacific Consultants.

He interrupted his teaching during World War II (1942-1946) when, as part of the 55th Naval Construction Battalion, Civil Engineering Corps. he spent two and a half years in the Seabees constructing overseas bases in the South Pacific, and a year and a half in research and development work with the Bureau of Yards and Docks in Washington, D.C.

He was the author of a dozen published works and a member of numerous professional organizations and fraternities, holding regional offices in many of them. His honors and awards include a Distinguished Service Certificate from the National Council of State Boards of Engineering Examiners (1957), Engineer of the Year award given by the Idaho Society of Professional Engineers (1959), Engineer of Merit commendation from the Spokane Section of the American Society of Civil Engineers (1965), and Outstanding Service to the Community award given by the Moscow Chamber of Commerce. In 1972 the new engineering building at the University of Idaho was named after him.

In 1947 he was elected to the Idaho State Board of Engineering Examiners to fill the unexpired term of Jesse Buchanan, who resigned upon assuming the Presidency of the University of Idaho. Janssen continued on this board until his resignation in May 1968.

He was also active in community affairs, serving as chairman of the Transportation Committee of the Chamber of Commerce (1969-1972), on the Board of Trustees of the Moscow School District (1948-1953), as a member of the Board of Directors for the Highway District, and on the Technical Advisor Committee for the Regional Airport (1969-January 1972). He was active in the Republican Party and was a member of the Sons of the American Revolution.

He married Ada Jones on July 14, 1930, in Logan, Utah; they had two children, a son Alfred and a daughter Sheila. Janssen suffered from Alzheimer's Disease and died at the Good Samaritan Village in Moscow on December 26, 1983.