Cromwell plays Andrew Mellon, then Secretary of the Treasury. At this point in the series he is relatively new to the job, having been appointed by President Warren G. Harding in 1921. He served under the next two Republican presidents too, until 1932. Then he was indicted and tried, but never convicted, by the Roosevelt administration.
In Season Three, we learned that Mellon owned the Old Overholt Distillery in Pennsylvania. The script writers have him conspiring with Nukie Thompson (Steve Buscemi) and Arnold Rothstein (Michael Stuhlbarg) to operate the distillery illegally. As Treasury Secretary, Mellon had partial responsibility for enforcement of the Prohibition laws.
Mellon, a banker, was one of the richest men in America. He was good friends with Henry Clay Frick, who made his stupendous fortune supplying coke to the steel industry. Frick was the grandson of Abraham Overholt, who had responsibility for his family's still on their farm near Pittsburgh starting in about 1800. By the time Frick came along toward the end of Abraham's life, Overholt's rye whiskey had made the family rich. Overholt employed many family members, including Henry Clay. After Abraham died, Frick muscled out the other cousins and took control.
We don't know if Mellon did any of the evil things depicted in "Boardwalk Empire," but we do know one thing he did that would be unacceptable today.
This was before the days of blind trusts and concern about conflict of interest. Mellon was considered a great man, a supremely successful businessman who chose to end his working life as a public servant and philanthropist. No one questioned it when, as Treasury Secretary, he granted himself a lucrative franchise in the form of a medicinal whiskey license.
The license allowed Old Overholt to legally sell existing whiskey stocks ‘for medicinal purposes only,’ but not to distill. Medicinal whiskey companies didn't make a lot of money, but the license was valuable. Because of it, Mellon was able to sell Overholt for a good price toward the end of Prohibition to Seton Porter, who was putting together what became National Distillers.
Old Overholt Straight Rye Whiskey is still sold today. It is made and owned by Jim Beam.