Boromir, as a character, is portrayed as having been born in the year 2978 of the Third Age to Denethor II and Finduilas, daughter of Adrahil of Dol Amroth. His younger brother, Faramir, was born in the year T.A. 2983. The following year, Denethor became Steward of Gondor, succeeding his father, Ecthelion II. Boromir simultaneously became heir apparent, inheriting the Horn of Gondor.
Boromir was named after the son of Denethor I, who was Steward around 500 years before the War of the Ring. The first Boromir was known as a great captain who cleared Ithilien of orcs of Mordor and was feared even by the Witch-king himself. Thus there were high expectations for Boromir the son of Denethor II.
When Boromir's mother Finduilas died (T.A. 2988), he was aged only 10. Denethor became sombre, cold and detached from his family. As their father withdrew, the relationship between Faramir and Boromir grew closer and greater in love. Denethor always favoured Boromir over Faramir—Denethor loved Boromir "too much, perhaps; the more so because they were unlike"—but this caused no rivalry between the two brothers. Boromir always protected and helped Faramir. Boromir was judged to be the more daring one, as well as the more fearless.
In response to prophetic dreams that came to Faramir and later to himself, Boromir claimed the quest of riding to Rivendell from Minas Tirith in T.A. 3018. His journey lasted a hundred and eleven days, and he travelled through "roads forgotten" to reach Rivendell, though, as he said, "few knew where it lay". Boromir lost his horse halfway along, while crossing the Greyflood at the ruined city of Tharbad where the bridge was broken. He had to travel the remaining way on foot and barely arrived in time for the Council of Elrond. (Tolkien wrote of Boromir's journey that "the courage and hardihood required is not fully recognized in the narrative".)