There are three men of importance named James in the New Testament.   First, there was the Apostle James, who was one of the “sons of Zebedee” (Matthew 4:21).  This James was beheaded by Herod (Acts 12:1-2). Second, James the son of Alphaeus (Mark 3:18), also was an Apostle, but he was not prominent in the New Testament books.  The third James (Matthew 13:55) would have been the natural child of Mary and Joseph and thus the “Lord’s brother” since they shared a common mother.  Initially, the Lord’s family did not believe in Jesus as the Messiah (John 7:5). However, they were eventually converted to Christianity as recorded in Acts 1:14.  This James became an important figure in the early church by both accepting Paul as an apostle and then by opposing Paul respecting Gentile conversions (Galatians 2:9-12). Paul also wrote, “Other of the apostles saw I none, save James the Lord’s brother.” [Galatians 1:19] The Greek for “save” means, “unless you want to consider” this James as one of the twelve Apostles, which he was not.

The history of this James, who wrote the epistle, is related by Eusebius in his Ecclesiastical History, Book 2, Chapter 1, written after 325 A.D.  He quotes the writings of the early Christian martyr Clement: “Then James, whom the ancients surnamed the Just on account of the excellence of his virtue, is recorded to have been the first to be made bishop [overseer] of the church of Jerusalem. This James was called ‘the brother of the Lord’ because he was known as a son of Joseph, and Joseph was supposed to be the father of Christ, because the Virgin, being betrothed to him, ‘was found with child by the Holy Ghost before they came together,’ as the account of the holy Gospels shows.  But Clement in the sixth book of his Hypotyposes writes thus: ‘For they say that Peter and James and John after the ascension of our Saviour, as if also preferred by our Lord, strove not after honor, but chose James the Just bishop [overseer] of Jerusalem.’  But the same writer, in the seventh book of the same work, relates also the following things concerning him: ‘The Lord after his resurrection imparted knowledge to James the Just and to John and Peter, and they imparted it to the rest of the apostles, and the rest of the apostles to the seventy, of whom Barnabas was one. But there were two Jameses: one called “the Just,” who was thrown from the pinnacle of the temple and was beaten to death with a club by a fuller, and another who was beheaded.’ ” 

Thus, by tradition and by the scriptures, it seems likely that James, the Lord’s natural brother, was the author of the book of James.