Harris was the eldest son of Christopher Harris of Hayne, Devon and Kenegie in Gulval, Cornwall and his wife Elizabeth Trott, daughter of Martin Trott of Langridge, Devon.
In 1691 Harris took his first leave of absence on grounds of ill-health, a pattern that continued throughout his parliamentary career.
At the 1698 English general election he was returned unopposed as MP for Okehampton, where his family had an interest, and was classed as a Country Whig.
He was returned unopposed for Okehampton at the two general elections of 1701 and by December 1701 was regarded as having become more or less a Tory.
The Whigs were unable to work out his political inclinations, but judged him a better choice than the Tory incumbent Thomas Northmore.