The Post Office at Shannock, (Richmond Township, Washington County) was established on October 6, 1884 under Postmaster Charles E. Sullivan. The Post Office is currently in operation under Postmaster Terry L. Place appointed on February 24, 2001.

The Post Office was originally established as Shamrock Mills, under Postmaster Charles Clark on June 26, 1839. It name was changed to Shannock Mills on January 22, 1861 and it became Shamock for 11 days on October 6, 1884. It's name was officially changed to Shannock on October 17, 1861. This appears to be due to clerical error on the part of Postal authorities in Washington, D.C.