John McGarrett was the father of Steve McGarret and Mary Ann McGarrett.
He was also the mentor of Lieutenant Chin Ho Kelly, and was a retired deputy of HPD (Honolulu Police Department).
In the pilot episode, he was murdered by Victor Hesse which in turn led to the creation of the Hawaii Five-0 Task Force as John's son, Steve sought to avenge his father's murder by killing Hesse.
Background
As an adult, John served in the Vietnam War, and became a cop.
Sometime during these years he met his wife (Doris McGarrett). Together they had two children: Steve -whom John named after his father, who had died while serving on board the USS Arizona-and Mary Ann.
On April 19, 1992, John's wife died in a car bomb made by the Yakuza. The bomb was revealed to be intended for him, leaving him to raise Steve and Mary alone.
Unknown to his children, John was investigating the Yakuza and Hiro Noshimuri but Noshimuri found out, and planted a bomb in John's car. Steve and Mary were told that their mother died in a car accident, killed by a drunk driver.
Fearing for his children's safety, John transported them to the mainland, without his children knowing his true intentions and as John never saw Mary, and possibly Steve, again.
With his children grown and gone, John decided to gather evidence of his wife's murder to convict Noshimuri. Then on September 20, 2010, John was taken hostage by Victor Hesse, and shot. John left behind a toolbox filled with evidence, which he wanted Steve to use to finish the investigation. It was revealed later that John's death was ordered by Wo Fat.
Despite being dead, John appeared as a ghost in the last few seconds of the Hookman (2010 episode) to thank Steve for stopping sniper Curt Stoner.
Personality
Like his son Steve, John is known to have a "laser focus." Although John deeply loved his children, Steve and Mary, he could not show it, therefore having a strained relationship with them. John also told a five year old Steve "to be anything but a cop." John's feelings from this statement are unclear, even though John himself was a cop.
John can also be described as tough, as he was not afraid when he was beaten or taken hostage by Victor Hesse. John also is shown to be fearless (also like his son), as he investigated Hiro Noshimuri, a very well respected and powerful man.