Who is Lady Mary's first husband?

Lady Mary's first husband is Matthew Crawley.

Matthew is a distant cousin of the Crawley family and becomes heir presumptive to Downton after Patrick Crawley dies in the Titanic disaster. A solicitor from Manchester, he initially finds the aristocratic world unfamiliar, while Mary resists the idea that her future may depend on marrying the estate's new heir.

Their relationship develops gradually through disagreements, separation and wartime hardship. Matthew serves in the First World War and is seriously injured, while Mary becomes engaged to Sir Richard Carlisle. After several reversals, Matthew and Mary reunite and marry in 1920.

Matthew dies in a car accident in 1921, shortly after the birth of their son, George. Mary later marries Henry Talbot, so Henry is her second husband, not her first. Patrick Crawley is another frequent mix-up: he was Mary's intended match and the previous heir, but he never became her husband. Tom Branson is Mary's brother-in-law through his marriage to Sybil.