His profession was never a question for Donald Flack. Since he comes from a family where all the men had taken on a police career, it was clear that Donald would be a cop too.

After high school and college, he attended the police academy and began his career in the New York Police Department, NYPD, shortly after. Donald has little contact to his younger sister, who also lives in New York and is suffering from an alcohol problem. His colleagues are surprised when they discover that Don speaks Gaelic, which he learned from his parents.

Like his colleague Jim Brass in CSI: Crime Scene Investigation and Frank Tripp on CSI: Miami, Detective Don Flack is the liaison between the NYPD and the Department of CSI: NY.

Donald Flack is an always well-dressed policeman, who is usually in good mood and is there to help his friends. He fondly follows basketball and hockey, loves a good glass of wine, and is allergic to cats.

As a policeman, he approaches his investigations in a very committed manner and even if Detective Flack would be quite willing to turn a blind eye on a small criminal mastermind and apply a certain level of understanding, he has a different kind of grudge against murderers, rapists and violence and capital criminals. This means that his work and procedures do not necessarily always go hand in hand with the rules of a policeman.

It has come to various differences of opinion between Flack and Mac Taylor. In a case of embezzled drugs, Taylor and his team had to investigate against a friend and colleague of Detective Flack. As part of their research, they spoke about it with Don, who held these ideas for ridiculous. However, all evidence pointed to the friendly police officer who was convicted of embezzlement and eventually drugs.

Although Detective Flack was initially furious about Taylor’s research, he arrested his guilty colleague. Nevertheless, the nature of Don Flack and Mac Taylor collegial relationship can be described as amicable.

Taylor is always ready to help Don in case of an emergency. When Don’s friend and colleague, Detective Jessica Angell is shot, he falls into a deep crisis caused by heartache and grief that which he cannot escape.

On the other hand, he is too proud to ask his friends for help, which means that he falls deeper into this cycle of anger and grief without anyone finds out. The result is him being more and more frequently drunk and coming in less regularly. He does not admit to any of his friends that he would have any problems that he can’t handle himself.

When he again leaves a bar drunk one night, he is ambushed and beaten. Fortunately, a petty criminal is there to help him. By locating Flack’s phone, Taylor manages to find and help him. In the course of the sixth season the detective attempts, however to get his life back on track.

Even Danny Messer from CSI: NY is good friends with Don and is ready to advise him when he will want to talk about his problems.