At St. Norbert, the Green Knights ended the 2009 season with a 9-1 record. The defense finished second in the conference and recorded two shutouts to conclude the season, marking the first shutouts since 1952. Prior to his stint at St. Norbert, Doornbos spent five seasons with the Northern Illinois Huskies.

After spending the 2003 season coaching the linebackers, Doornbos was promoted to defensive coordinator and linebackers coach and held the position with the Huskies until 2007. In 2003, NIU became the first team in the Mid-American Conference (MAC) to defeat three BCS Schools en route to a 10-2 overall record. The Huskies improved on that success in 2004 and finished with a 9-3 record and won the Silicon Valley Bowl. The 2005 Huskies won a second straight MAC West Division title and finished atop the conference among scoring defenses. NIU closed the 2006 season with an appearance in the Poinsettia Bowl against TCU.

Doornbos, who has over 30 years of coaching experience, also spent 13 years at the United States Military Academy where he was defensive coordinator and defensive ends coach for nine years. He also spent time on the coaching staffs of University of Texas at El Paso, University of Michigan, Kent State, Boston College, University of Illinois and Western Michigan, where he received a bachelor of science degree in 1978.

Doornbos is married to Helen and they have two sons, Blaik and Darik.