Is it true that the Sheriff is the only law enforcement agency actually mentioned in The Constitution?
Nope. Not in the FEDERAL Constitution, but, the role of Sheriff is a position created by *state* legislators or the general assemblies within the individual state.
Might even be in the Constitution of one to several States. Dunno.
In the United States, a sheriff is a county official and is typically the top law enforcement officer of a county, answerable to only those who elect him. The Office of Sheriff is NOT in the US Constitution, only in some state constitutions.
USUALLY, but surprisingly, the ultimate LEO in may counties is the Coroner.