According to Kirk, he first became interested in forensic science in his early days as a teacher after a student asked if it could be determined if a dog had died of poisoning.

Forensic science permanently piqued his interest when authorities asked him to do a microscopic examination of a rape victim's clothing.

Due to his extensive experience in microscopy, Kirk was asked to join the Manhattan Project where he worked on the process to isolate fissionable plutonium from 1942 to 1945.

On January 22, 1955 – one month after Sheppard's conviction – Kirk visited the scene of the crime after which he wrote an extensive report primarily based on bloodstain pattern analysis.

Kirk's testimony embarrassed Samuel Gerber, a senior member of the American Academy of Forensic Sciences (AAFS) who was working for the prosecution and believed in Sheppard's guilt.