It lies in the north-eastern part of Covasna county, along the river named RiuI Negru, at the meeting point between the hilly zone of the Nemira Mountains and the Targu Secuiesc Depression. The lower part of the commune is crossed by the E 574 (DN-11) international road. Lemnia has a railway station on the Sfantu Ghe-orghe - Bretcu secondary line.
It is situated at a distance of 11 kms from TTrgu Secuiesc and at a distance of 41 kms from the county capital. It is 9 kms long and it is divided into 3 parts, namely Lemnia de Jos, Lemnia de Sus and Velence. Although regular excavations weren't carried out in this region, there are data which prove that finds originating from the Gothic civilization have been discovered.
It could be a known settlement already in the early Middle Ages since on the Varerosse Mountain there is an early medieval castle named Almas. It can be approached along the Lemnia brook, only its ruined basement walls can be seen today.