Chapman was a Cornet in the regiment of cavalry raised by Murrough O'Brien, 1st Earl of Inchiquin.
After the conquest, Oliver Cromwell granted Chapman an estate in the county of Westmeath, at Killua (also known as St. Lucy's), a former preceptory of the Knights Hospitallers.
[1] Chapman's father, William, and uncle, John, had been supported by their maternal first cousin, Sir Walter Raleigh, who helped them procure land grants in the County Kerry.
Due to financial troubles and their patron Raleigh's death in 1618, they were forced to sell the land to Richard Boyle, 1st Earl of Cork, for £26,400.
William remained with his father at St. Lucy's and married Ismay, daughter of Thomas Nugent (esq.