According to Clara LaGuerta, Harold cheated on his wife, Elizabeth, with his children's nanny (Clara herself). He later kicked her out with no money or shelter. After meeting her, Clara asks Marston to get some money for her, and when confronted with the accusations, Harold denied them and challenged Marston to a duel, resulting in Harold's death.
When told of Harold's death, Clara cried and said she could still make it to the funeral. A few days later, Marston met Elizabeth, Harold's widow, in the cemetery. Elizabeth mentioned that Clara was laughing at the funeral and bitterly moaned about Clara that "strangers can be so cruel". This implies that not only Clara did not have an affair with Harold, but that she never even worked for the Thorntons, which also implies that she tricked Marston into murdering and robbing an innocent man. This is confirmed by the mission summary on the player's Journal.
The player meets him in the Blackwater Saloon and he inevitably challenges the player to a duel. The player can later find him at the Blackwater Cemetery where he will speak with Harold's widow.
- As with all mission Duels, Marston must kill Thornton to win; attempting a disarm will result in Marston dying.
- Considering his thick accent (and his last name), it is most likely that he is a Scottish immigrant.
- He and Uriah Tollets share the same voice actor.
- On his grave is the writing: "THE TRUTH SHALL MAKE YOU FREE" this is of similar wording to the mission: "And The Truth Will Set You Free".
- It seems that Harold uses a rather unique version of the Double-action Revolver . This being that it has the same model as the original one but it is made out of shiny steel, similar to the Cattleman Revolver. This version of the revolver is, however, not obtainable and when picked up will only give the player a small amount of ammo.
- When Clara asks Marston to get money for her, she does not give him any details about Harold's likeness or even his exact location. The only clues she gave were the fact he was known to be a drinker, and he lived in Blackwater. When Marston walks into the saloon he boldly walks up to the man leaning against the bar. He does not even have any proof that the man he dueled was Harold until he sees his wife at the cemetery days later. Marston must have just assumed that he was in the saloon because of the clues she gave him.
- There is a glitch sometimes when Harold walks to get to the duel, he will be stuck in one spot and it will say on the HUD "Stranger Mission Paused". This glitch can be remedied by simply saving in the bed in the saloon.
- There is a glitch where the player can keep the $200 obtained from Harold and do it repeatedly. First after meeting him don't accept the duel challenge from Harold. Instead lasso him, hogtie him, then put him on a horse. Run outside of town and kill him. Killing him inside of town results in a message to loot his body and give the money to the girl, but outside it does not count towards the mission. After looting the $200 from him go back into town and he will respawn so that the mission can be repeated.