Mountstephen was the second son of John Mountsteven and Elizabeth (relict of Tamlyn) of St Mabyn, Cornwall.
Mountstephen purchased the barton of Lancarffe (Helland) in 1685 from the Bullock family while secretary to the Earl of Sutherland.
[1] His brother, William inherited Lancarffe, after a jury returned a verdict of lunacy following John's suicide.
On 19 December 1706, Mountstephen was at Brown's Coffee House, King Street, Westminster in the company of other MPs when he took out a razor from his pocket and slit his throat.
His suicide was probably because it became public about his passing of state secrets to the King of France, in return for a pension, although there were rumours that he took his life after his proposal of marriage was rejected by someone of higher social standing.