The family may not be entirely poor, since one of Tom's grandfathers was a tenant farmer in the county of Galway, specializing in Blackface sheep. Tom was born in Bray, County Wicklow. However he seems to have spent most of his life in the city of Dublin, having told Matthew Crawley he isn't much of a country boy. Nevertheless he did spend time at his grandfather's farm, mentioning to Lady Sinderby he shot pigeons there as a boy.
Mrs Branson, who lives in Dublin, has at least two children: Tom and Kieran. Both have worked with cars: Kieran being a car mechanic based in Liverpool, and Tom having worked as a chauffeur in both Ireland and England.
Tom has more than one cousin, indicating his mother and/or his father may have at least one sibling. A male cousin was murdered by an English officer during the Easter Rising, "because he was probably a rebel." It is likely then that the majority of the family hate the English. Another male cousin has settled in Boston, selling cars and farm equipment. Tom later tells Matthew of the possibility of hiring a woman or getting "a cousin over" to look after his child, suggesting he has at least one female cousin. One of them, Nuala, had an illegitimate daughter brought up as her sister to protect her reputation.
Tom is well-read, interested in history, and has strong socialist and republican political views. After being hired as chauffeur to the Earl of Grantham, he becomes very drawn to the earl's youngest daughter, Lady Sybil Crawley, a woman likewise politically minded and caring for the less fortunate.
Despite his love for Ireland and his opposition of the English and the aristocracy, Tom falls in love with Sybil and remains in England for years before she agrees to marry him against her father's wishes. They wed in 1919, and live in Dublin for roughly a year until in 1920 they are forced into exile in England after Tom is suspected of being involved in the torching of an Irish noble's estate. He is devastated, but remains at Downton Abbey.
Lady Sybil dies soon after giving birth to a daughter. Tom names the baby Sybil, after her mother, but she is commonly referred to as Sybbie. She is baptized at the Catholic church in Ripon, attended by Kieran and her maternal family.
Tom initially considers leaving with Sybbie but remains after being hired as the estate's agent. Over time, his life and interests (including his politics) seem to fade away. He decides to move to Boston, Massachusetts with his daughter in January 1925, but eventually they return to live at Downton for good. Tom eventually starts up a car dealership with his brother-in-law, Henry Talbot.
- It is unknown if Tom's father is still alive, as well as how many cousins and siblings he has. It is known, however, that his mother and at least one brother are alive, and that he did, at one point, meet one of his grandfathers who was a sheep farmer (though it is not known if he is alive or if he is Tom's paternal or maternal grandfather).
- It is unknown of Tom's cousin Nuala is his paternal or maternal cousin. If paternal then her name would be Nuala Branson.