He worked for the Standard Oil Company of New York from 1910 until he was called to active duty through the Officer Reserve Corps (ORC) in 1917.

In 1933, Andrus commanded a Civilian Conservation Corps Camp in Oregon and on January 1, 1934, he was assigned as plans and training officer, 13th Cavalry, until July 1934.

He then served as a board member at the United States Army Infantry School to develop and implement a cohesive air-ground cooperation, and aerial recognition and identification programs.

On October 10, 1942, Andrus was transferred to Caven Point Terminal, New Jersey as commandant and on January 1, 1943, to Brooklyn Army Base as the officer in charge of the Control Branch.

He held this position until August 28, 1943, when he was reassigned to be director of intelligence, Security Division, New York Port of Embarkation.

Transferring to Europe, Andrus was assigned on January 27, 1944, as commanding officer of the 10th Traffic Regulation Group (TRG).

While assigned to the 10th TRG he was on detached duty as liaison officer with the 21st Army Group (British) from August 22 through December 10, 1944.

Colonel Andrus was a strict disciplinarian who made no distinction between those Nazi leaders who were military or civilian, treating them all as war criminals.

Albert Speer commented (in his book Inside the Third Reich) that Andrus cordially greeted him when he arrived at Nuremberg Prison and also briefly mouthed an apology for having to maintain strict discipline.

Göring had mounted an arrogant coordination of the prisoners and often demagogued in court; Andrus' food regimen saw him shrink down by 80 lbs.

After the suicide of Dr. Robert Ley, Andrus arranged anti-suicide cells in which even the tables were designed to collapse under a man's weight.