Mr William John Cassidy was born in Belfast, Ireland around 1881. He was the son of George Cassidy, a labourer, and grew up in a Presbyterian family. 

He was married in Great Victoria Street Church, Belfast on 27 December 1904 to Ellen Johnston (b. circa 1882). By 1911 the couple lived at 42 Bright Street in east Belfast and by then had three children: Mary Ellen, George and Isabella. 

Cassidy’s ship prior to Titanic had been the Olympic. He joined Titanic at Belfast for the delivery trip to Southampton where he then disembarked.

In September 1912 William Cassidy was a signatory of the Ulster Covenant, a petition signed by nearly 500,000 citizens opposing Irish Home Rule. 

William Cassidy remained at 42 Bright Street for the rest of his life. He died in a Belfast sanitorium on 28 December 1953 aged 72 and is buried in Dundonald Cemetery (plot C4 735). His widow Ellen died four years later and they are buried side by side.