Tom Branson (born in or before May 1890[1]) is an Irish Republican, the husband and widower of Lady Sybil Branson née Crawley, son-in-law of Robert Crawley, Earl of Grantham and Cora Crawley, Countess of Grantham, brother-in-law of Lady Mary Crawley née Crawley, Lady Edith Crawley and the late Matthew Crawley, father of Sybil Branson, and he is also related to the Branson family.

Hired to be the Crawley family chauffeur, he inspires Lady Sybil Crawley to get involved in political causes, and over time they develop a romantic relationship, later marrying despite the reservations of her family, and living in Dublin, Ireland where Tom gets a job as a journalist. By Christmas 1919, they are expecting their first child and in 1920 return to Downton to visit the Crawley family and attend Mary's wedding. Months later, Tom unexpectedly arrives at Downton after fleeing Ireland without Sybil, leaving her to arrive the following day. Due to the trouble he is in, Tom has been forbidden from returning to Ireland, so he and his wife begin living at Downton. Sybil later gives birth to their first and only child and daughter, but dies shortly afterwards of eclampsia with Tom and her family at her side, leaving Tom, the Crawley family and their staff shocked and heartbroken. Tom names his daughter after his late wife.