Master of Coin is a title in Game of Thrones. The holder is a member of the small council of the monarchy of the Seven Kingdoms, who advises the king on financial matters. The Master of Coin is in charge of the royal treasury, and responsible for filling it with money to meet the crown's needs.
In the A Song of Ice and Fire novels, Petyr Baelish was appointed Master of Coin by Hand of the King Jon Arryn, about seven years before the beginning of the narrative. Originally, Jon's wife Lysa had convinced her husband to make Baelish the chief customs officer at the port of Gulltown in the Vale. Baelish excelled at the position, increasing revenue tenfold, so ten years ago Lord Arryn brought him to King's Landing and gave him a minor position in the royal customs office. Baelish increased gross revenues tenfold, and demonstrated such great skill that Arryn rapidly advanced him through a series of promotions, so that within three years he was named Master of Coin for the entire realm. Robert's reign lasted fifteen years in the books (expanded to seventeen in the TV series), and Littlefinger was only Master of Coin for the last seven years, but it is unstated which person (or persons) may have held the office in the early years of King Robert's reign. Thus Baelish is not a holdover from the previous Targaryen Small Council, in contrast to Pycelle and Varys, who formerly served under the Mad King.