The package was found on Bel Air Road, near Mnuchin's home, in Los Angeles. The Secret Service was also involved in examining the horse excrement, because of Mnuchin's role as a cabinet member for President Donald Trump.
A suspicious package has been tracked outside the home of US Treasury Secretary Steven Mnuchin in California.
An LAPD bomb squad X-rayed the package before opening it and found the horse manure inside, police told local media.
The package also included a Christmas card with negative comments about President Donald Trump and the new US tax law signed by Trump last week. Police said the package had been gift-wrapped and was marked as being from "the American people".
Mnuchin has served as Trump's finance chief during the presidential campaign.
Treasury Sect'y Steve Mnuchin and wife Louise must have been naughty this year - as some say - because he didn't even get coal for XMAS. he got a package of s***. He previously worked in banking and finance before joining public service, acting as CEO of Dune Capital Management and working for The Goldman Sachs Group earlier in his career.