Born at The Hague, son of Captain John Day and his wife Emilie (née Hartsinck), Day completed his schooling at Downside, and, as the Universities of Oxford and Cambridge were at that time inaccessible to Catholics, obtained his BA from the University of London in November 1845.

He was admitted as a student in the Middle Temple in 1845, was called to the Bar, and became a judge in June 1882.

At the time it was said that it was on his insistence that early proof was tendered of the authenticity of the letters attributed to Parnell, which forced Richard Pigott into the witness box and led to the collapse of that part of the case.

One of the most notable is a very large painting that is on display in the Royal Courts of Justice in London of several judges including Sir John Day.

The Times ran an obituary notice on 18 June 1908, and carried an article on the sale of his art collection on 14 May 1909.