Mr. Mason is the father of William Mason and father-in law of Daisy Mason. He owns a farm. Despite his son's wish to be a groom (due to his natural affinity with horses), Mrs. Mason was determined that her son would one day be a butler and so he became a footman to please her. Even as she was near death, she refused to let William know as she thought his job was more important. Mary, noticing William's kind nature, decided it was wrong and told him to visit his mother as soon as possible although she did not tell him how grave his mother's illness truly was. William was able to say his final farewell to his mother before her death, but still remained very upset over it. It is unknown how Mr. Mason reacted. With the outbreak of World War I, William is eager to serve but his father refuses because William is an only child. But after being presented with a white feather and support from Isobel Crawley, William finally gets to enlist. It is not known how his father reacted to this. William becomes engaged to Daisy and is sent to be a servant to Matthewon the front lines. However, both Crawley and William are injured and while Crawley's injuries render him paralysed from the waist down, William's are proved to be fatal and he is brought back to Downton to die slowly but in the company of his family from lung failure. He also saved the life of Captain Crawley by taking the shell which then killed him. During this time, he marries Daisy before passing peacefully away on the 26th July 1918. His father arrived at Downton Abbey for the wedding. Mr. Mason was next scene speaking to Daisy at William's grave following Lavinia Swire's funeral. He remarks that there is nothing more terrible when young people die. He tries to contact Daisy and does have a drink with her, as Mrs. Hughes explains, "You're all he's got Daisy."
"Now you're special to me." - Mr. Mason to Daisy.
Daisy is uneasy about this but she later goes to visit him at his farm, having promised to do so. Here Mr. Mason tells Daisy she's the nearest thing to a child he has, and that William had three brothers and a sister who died young. He believes one reason William married her was so he would not be alone, because "without you I'd have no one to pray for." From this point on, Daisy and Mr. Mason agree to support one another as father and daughter, Daisy having no parents of her own and never being special to anyone, "except William" as Mr. Mason makes her realize. Then he says she is special to him. He then gives her a ride away from the farm on her way back to Downton Abbey in order to talk to her, and gives her some encouragment and the confidence to ask Mrs. Patmore for a promotion.
When Daisy revisits his farm in 1920, he offers to name her his heir and even asks she move in with him so he can teach her how to run the farm. She is uneasy about this, thinking she would spend her entire live in service. To that Mr. Mason remarks estates like Downton are slowly going away.