Note : dates are given using the New Style calendar.
He was apparently born in Gloucestershire. It is not clear who his parents were: Burke's Commoners makes him out to be the son of Lawrence and Anne Holliday of Gloucester, but the children listed in Lawrence's will (1587) and the brother and sister named in William's will (1623) do not have the same names, so they appear to be different families. Nevertheless, according to William's will, he was a cousin of Sir Leonard Holliday, who was later Lord Mayor of London.
William was later sent to London, probably in the 1580s, where he served an apprenticeship in the Worshipful Company of Mercers, which controlled the city's textile industry. In 1594 he took part in the Duke of Cumberland's expedition to the Azores which involved the sinking of the great Portuguese carrack the Cinco Chagas.
He married circa 1600, Susan (or Susannah), the daughter of Henry Rowe, a prominent member of the Worshipful Company of Ironmongers and later Lord Mayor (whose father, Sir Thomas, had been an earlier Lord Mayor). William and Susan had two daughters: Anne (born in 1602) and Margaret (born circa 1603).
William evidently had a successful career as a mercer. He was also involved in the East India Company, of which his cousin Sir Leonard had been a founder, and was elected as a 'committee', i.e. director, in 1616.
William's stature and success were reflected in the arranged marriages of his two daughters. In April 1619, after "long and ernest sollicitation" by the Marquess (later Duke) of Buckingham, who was acting on instructions from King James, he married off Anne to Sir Henry Mildmay. In February 1620, William married off his younger daughter Margaret to Edward (later Sir Edward) Hungerford. They later lived at Farleigh Hungerford Castle, and are buried there.
In 1621, William was elected governor of the East India Company, a position he held for the remaining three years of his life.
William died on 14 February 1624 and was buried in or near St Lawrence Jewry church. He left a substantial estate: around £40 000 in cash, and properties in London, Middlesex, and Gloucestershire. His widow Anne later married the Earl of Warwick. She died in 1646.
Many years later, their daughter Margaret erected a monument to William, Anne, and her sister Anne, in St Lawrence Jewry church. It was irreparably damaged during an air raid in 1940.
Coat of arms
Shortly before his death in 1624, William obtained a grant of arms and crest from the College of Arms. The arms were: Sable, three helmets Argent, a bordure of the same, i.e. a black shield displaying three silver helmets all within a silver border. The crest was the upper half of a gold lion, looking straight ahead, and holding an anchor. These arms and crest bear some resemblance to those of Sir Leonard Holliday, the differences being in the trimming of the helmet, the edging of the border, and the direction in which the lion faces. 
- Burke, J. (1836). A Genealogical and Heraldic History of the Commoners of Great Britain and Ireland Vol II
- Holladay, A.> (1983). The Holladay Family
- Cokayne, G.E. (1897). Lord Mayors and Sheriffs of London 1601-1625
- William Holliday's Last Will and Testament
- Smith, C.F. (1901). Mary Rich, Countess of Warwick
- Pope-Hennessy, J. (1941) History Under Fire
- Edmondson, J. (1780) A Complete Body of Heraldry; Burke, Sir B. (1884) The General Armory'.