The Commune was originally called ‘Saint-lunan’ (Saint Breton). The name ‘Saint-Aignan’ was adopted at a later date. Aignan was the bishop of Orléans in the VI^th Century and is renowned for fighting against Attila and the Huns. The name ‘Aignan’ or ‘Agnan’ can be found elsewhere in the names of over thirty towns and villages throughout France. Saint-Aignan was formerly included in the ancient parish of Cléguérec. The bishop of Vannes, Saint Morvan, was probably buried in the commune of Saint-Aignan in the VII^th Century.

The name ‘Saint-Iunan’ was mentioned by Alain III de Rohan as far back as 1184 in the Foundation Charter of the Abbey Bon Repos. Small domains such as La Villéon and Du Fou can be found in the Ancien Régime. Initially part of the ancient parish of Cléguérec, Saint-Aignan was established as a parish in its own right in 1802 and then as a Commune in 1790.

The feudal site of Motten-Morvan was set up at Corboulo, the area where, it is believed, that the Knights Templar established themselves. (Listed as an historical monument in 1995)

The original camp of Castel-Finans (Listed as an historical monument in 1971): This site, remarkable for its position dominating the Guerlédan barrage was one of the homes of Konomor, a powerful Lord who reigned over a large part of Brittany in the VI^th Century. Today the only visible remains is a ring of stones encircling the site of about 4 hectares. Buildings at the time (even those of the most powerful) were still being constructed in wood. A piece of work by Gilles Rihouay about Konomor and Castel-Finans has been published by Montagnes Noires.