Mather Brown was born into a wealthy merchant family based in Waterdeep. Historically, his family were weaponsmiths, although the last 4 generations had earned much greater wealth in trading magical arms and armor. He was a second son, and his older brother, Corman, was pegged to take over the family business. Mather was expected to be a good second son and enter into a profession in the clergy or the military.
As per family tradition, he served an apprenticeship in weaponsmithing. After he completed his apprenticeship at the age of 18, he joined the Waterdeep Watch as a Lieutenant. Mather was too much of a free spirit, which did not mix very well with regimented military life. After an 8 month stint in the Waterdeep Watch, he was asked to leave.
He then took a job as a caravan guard, and traveled extensively. Although the life of a guard was less regimented than the life of a watchman, his carefree attitude and loose adherence to the rules annoyed his superiors greatly. As his reputation as a "spoiled rich brat" began to spread it became harder and harder for him to find work. Eventually he abandoned the life of a caravan guard as well and became a free-lance adventurer, hunting treasures and helping those in need, as needed. Mostly however, he spent his time in Taverns and Inns around Waterdeep, drinking, singing, and womanizing. He became a bit of an embarrassment to his family, but they tolerated him as long as he did not overdo it.
This all changed in the next year. His father, Roland, had arranged a marriage between his brother, Corman, and the daughter of another wealthy merchant family. Lorelai Lee was a pretty young girl who resented being forced to marry a man she had never met. She arrived in Waterdeep with her entourage, in order to meet her intended and become acquainted with him before the ceremony. Mather and Lorelai met at the introduction party, and although they had little time to spend together, they exchanged frequent glances and smiles.
Later that evening, Lorelai got away from her escorts and tracked Mather down to his favorite tavern. They spent the night drinking and singing together and enjoying each others company. They rented a private room, and spent the night in each others arms.
Unfortunately, Lorelai's maidservant tracked her down, and caught the two lovers in bed together. The next day there was an uproar. Camber refused to marry a girl who had been soiled by his own brother. Lorelai's father argued that, since the Brown family was responsible for his daughter losing her virginity, the marriage would have to go forward despite Camber's objections.
Mather's comment that the girl was most certainly NOT a virgin did nothing to ease the tension.
In the end, Corman was forced to marry Lorelai, and the Brown family suffered from a great deal of embarrassment among the nobility. Mather was asked to leave Waterdeep and never return. Mather agreed to do so. He packed his few belongings and left for Sundren within the week.