The church of St. Garmon ([[St. Germanus]]) is thought to be of early-mediaeval origin; it was 'restored' in 1886 to designs by W. H. Spaull of [[Oswestry]].<ref name=llanfylin>[http://llanfyllindeanery.org.uk/parishes/mynyddmawr.htm St Garmon's Church, Llanarmon Mynydd Mawr]</ref> The area was historically part of the parish of [[Llanrhaeadr-ym-Mochnant]] but eventually became a separate [[township (England)|township]] for administrative purposes.<ref name=cpat>[http://www.cpat.org.uk/projects/longer/histland/tanat/tnadmini.htm Tanat Valley], Clwyd-Powys Archaeological Trust]</ref> It is now part of the [[community (Wales)|community]] of Llanrhaeadr-ym-Mochnant.
The area of the parish is today largely mountain pasture. There are a number of rare late-mediaeval [[cruck]]-framed buildings.<ref name=cpat2>[http://www.cpat.org.uk/projects/longer/histland/tanat/1021.htm Tanat Valley Historic Lanscape Characterisation], Clwyd-Powys Archaeological Trust</ref>
The academic [[Griffith Hartwell Jones]] was the son of a rector of Llanarmon Mynydd Mawr, the Rev. Edward Jones.