The origin of this name is unclear. It may come from the Gaelic name "Fearsithe" meaning "man of peace" or it may refer to a place or lands. Some say it is is decended from Forsach, a Norsemen who settled in Auitaine. In 1296 Robert de Fauside signed the "Ragman Roll". After military defeat, men were made to swear allegiance to Edward I of England and sign the "Ragman Roll".

After 1306, Robert de Forsyth's son Osbert, received a grant of land from Robert the Bruce at Stirlingshire. In 1368 Osbert's son was appointed the king's macer and constable of Stirling Castle. After that, the family became established around that area.

There was a Forsythe castle at Dykes where David Forsyth of Dykes aquired his land, but it was demolished in 1828. After that, many of the family moved, spreading it throught Ayrshire and Glasgow.

Some records were lost during transportation to England because of war in the mid 17th century and the then chief didn't re-register his claim to the name when Charles II instituted a Public Register of Clans in 1672.

The present chief was recognized by Lord Lyon in 1970 and he's since restored Ethie Castle near Arbroath.