"We have more that we are planning to do if they don’t release him quickly," Mnuchin said during a meeting of President Donald Trump's Cabinet.
The United States and Turkey have exchanged tit-for-tat tariffs in Trump's attempt to persuade Turkish President Tayyip Erdogan to give up Brunson, who denies charges that he was involved in a coup attempt against Erdogan two years ago.
"They have not proven to be a good friend," Trump said of Turkey during the Cabinet meeting. "They have a great Christian pastor there. He's an innocent man."
The dispute over Brunson and other frictions between Washington and Ankara have been one reason the Turkish lira has plunged 40 percent this year. Investors also fret over Erdogan's influence over monetary policy.
The lira lost strength after Mnuchin's remarks.
Trump said the United States had helped Turkey in a related incident, but that the NATO ally had not repaid the favor.
"They want to hold our wonderful pastor. Not fair, not right," he added.
Trump, who doubled steel and aluminum tariffs on Turkey, said the steel tariffs have kicked in the aluminum tariffs will take effect soon.
(Reporting by Steve Holland; Writing by Tim Ahmann; Editing by Chizu Nomiyama and Alistair Bell)