Elected for Cornwall in 1625, he did not stand again but his extensive estates gave him influence over the Parliamentary seats of Grampound, Tregony and St Mawes.

[1] Although Trevanion supported Parliament against Charles I in the 1628 debate over the Petition of Right, in May 1641 his son John was one of 59 MPs named as "betrayers of their country" for voting against the Bill of Attainder for Strafford.

In August 1643, he succeeded Sir Nicholas Slanning as Vice Admiral of South Cornwall, after the latter was killed along with his son John at Bristol.

This was a largely nominal post, since the Parliamentarian navy dominated the sea lanes, but Trevanion also raised an infantry regiment.

[4] During the 1648 Second English Civil War, Trevanion was suspected of involvement in an abortive Royalist rising in Cornwall, and briefly arrested in 1650.