He was the son of William Helyar and Alice Veale and was baptised on 8 January 1559 at St. Budeaux, Devon. He married Mary Cary, daughter of William Cary. 
The children of William Helyar and Mary Cary included
- Elizabeth Helyar (christened 12 May 1591, married John Bridgeman, son of Thomas Bridgeman and later Bishop of Chester, on 29 April 1606)
- Henry Helyar (m.1621 to Christian Cary, died 1634), whose son William Helyar (died 1697) succeeded his grandfather to the manor Coker Court.
- Vicar in 1577 at Bickleigh, Devon
- Rector in 1582 at Dunchideock, Devon
- Rector in 1588 at Charlton, Devon
- Canon of Exeter Cathedral in 1596
- Vicar in 1602 at Heavitree, Devon
- Archdeacon of Barnstaple from 1605
- Rector at North Tawton, Devon, between 1610 and 1645
He purchased Coker Court, the manor of East Coker in 1616 from Sir Edward Phelips. He started the construction of the Helyar Almshouses in East Coker in 1640, but died before their completion. They were finished by his grandson (also William Helyar).
On 19 January 1642, during the English Civil War he was dragged from his bed at midnight in Exeter by Roundheads and carried on board a ship, where he was detained until he consented to pay the sum of £800 (£117,521 as of 2014),. He received a copy of the protection that he purchased
Whereas Archdeacon Helyar has submitted and sent eight hundred pounds to the parliament upon the propositions. These are therefore, to require all the forces of the parliament to take notice thereof and to protect him in his person, family, and goods, strictly charging and commanding that no man henceforth, molest or trouble him. January 19, 1642, signed, Jo. Northcote, Sam. Rolle” etc etc
- The Complete Baronetage. Alan Sutton Publishing, Gloucester, 1983, volume 3, page 27.
- Burke's Genealogical and Heraldic History of the Landed Gentry, 15th edition, (London, England: Burke's Peerage Ltd, 1937), page 1088.
- Batten's South Somerset Villages, John Batten, F.S.A, Somerset Books, 1994. ISBN 0-86183-270-1
- UK CPI inflation numbers based on data available from Gregory Clark (2013), "What Were the British Earnings and Prices Then? (New Series)" MeasuringWorth.
- A genealogical and heraldic history of the commoners of Great Britain and Ireland, enjoying territorial possessions, or high official rank, but uninvested with heritable honours. John Burke. Colburn, 1834