He finished vocational school in 1966, undertook a three-year apprenticeship in typography and worked as a typographer in Arbeiderbladet from 1969 to 1979 and Verdens Gang from 1979 to 1980.
In 1980 he became a national board member of the Norwegian Graphical Union, and in 1984 he became leader of the Norwegian Graphical Union and Oslo Graphical Union.
From 1987 he was also a supervisory council member in the Norwegian Confederation of Trade Unions, and from 1988 he was a board member of Oslo faglige samorg.
He rescinded all positions in 1989 to become an elected politician.Kristensen was elected as a deputy representative to the Parliament of Norway from Oslo during the terms 1989–1993 and 1993–1997.
However, he met as a regular representative during seven of the eight years, as a stand-in for Gro Harlem Brundtland who served as Prime Minister; later for Thorbjørn Berntsen who took part in Jagland's Cabinet.
In the 1997 Norwegian parliamentary election Kristiansen was finally elected as a full member, and served through 2001.