Lillehaug was born in Waverly, Iowa and raised in Sioux Falls, South Dakota, where his father taught music.

[2][3] He attended Augustana College in Sioux Falls and graduated in 1976 with a Bachelor of Arts summa cum laude.

[4] Lillehaug was appointed by President Bill Clinton to serve as the United States Attorney for the District of Minnesota beginning in 1994.

[8] On March 16, 1999, Lillehaug announced that he was entering the 2000 race for the United States Senate DFL nomination in Minnesota.

[12] On June 12, 2019, Lillehaug announced a diagnosis of Parkinson's disease and his retirement from the court effective July 31, 2020.