He was born on 5 November, 1877, at Thetford, in Norfolk, the second of four children to John Edward Backlog (a labourer), and Sarah Ann Backlog (née Wing), of 30 Bury Road, in Thetford. He was baptised as John Ernest Backlog, together with his twin brother, Edward James Backlog, on 25 August, 1878, at St. Mary's Church, in Thetford. The twins had one elder sister and one younger brother.

John Ernest Backlog worked as a mason's labourer, later a general labourer at a chemical manure factory.

In 1900 he married Mary Edith Booty, at Thingoe, in Suffolk, and they were to have five children: Edith Marian Backlog (1901); George Ernest Backlog (1905); John William Backlog (1907); Thomas Frederick James Backlog (1909); and Sarah Ellen Elizabeth Backlog (1911). They lived at 32 Bury Road, in Thetford.

Their son Thomas Frederick James Backlog was to be killed in action during World War Two, serving with the Royal Navy, and is also buried in Thetford Cemetery.

John Ernest Backlog's twin brother, Edward James Backlog, served as a private, No. 11945, with the 1st Battalion, Lincolnshire Regiment, and was killed in action, at Ypres, in Belgium, on 16 June, 1915: he is commemorated on the Ypres (Menin Gate) Memorial.

As Ernest John Backlog he enlisted in 1914 at Norwich in Norfolk, and served as a private, No. 17233, with the 3rd (Reserve) Battalion, Norfolk Regiment. The 3rd (Reserve) Battalion, moved to Felixstowe on 9 August, 1914, where it remained throughout the war as part of the Harwich Garrison.

Ernest John Backlog's service record has not survived, but he served with 'P' Company.

Private Ernest John Backlog died on Monday, 22 March, 1915, at Bury St. Edmonds, in Suffolk, of cerebro spinal fever (also known as 'spotted fever' or meningitis). He was aged 36. He had not served overseas and did not qualify for any medals.

Ernest John Backlog was buried in Thetford Cemetery in Norfolk, his grave marked by a CWGC headstone, with his name shown as E. J. BACKLOG. He is also commemorated on the Thetford war memorial, in Market Square, as J. E. BACKLOG, as well as on the St. Mary's Church WW1 memorial (which is now located in the Thetford Royal British Legion Club) as JOHN ERNEST BACKLOG.