Grenfell was accidentally killed when a water tank he was carting fell on him when the shaft of the dray broke. An inquest was held at Goongarrie on 18 November 1918 where the verdict of the jury was: Accidental death caused by a tank of water falling on him whilst carting water, the shafts of the dray breaking, with no blame attached to anyone."
Grenfell was the ninth of ten children born to Richard and Ann, who married in Cornwall, England, on 16 February 1839 and who travelled to Australia in 1849 with Mary Ann born 1837; Richard born 1839; Nicholas born 1842 (died 1843); Nicholas born 1844; Cyprian Nicholls born 1847. The family arrived in South Australia on 3 January 1849 aboard the "William Money". Four children were added to the family in South Australia. They were: Thomas Henry born 26 April 1849; Sarah Jane born 31 January 1851; Elizabeth Warren Ruth, born 7 August 1852; William James born 1854; Jane born 1 August 1859. Richard died in Clare, South Australia in 23 February 1866.
Ann died on 17 February 1897 in South Australia. William James' brother, Richard, died in Kalgoorlie in 1909 and was reported as "an old pioneer". He may have been in Coolgardie in 1893. Brother Nicholas was also at Goongarrie in 1913, and Thomas at Sandstone in 1910.