In preparation for a career in the law, Northey was educated at St Paul's School and matriculated at Queen's College, Oxford on 4 December 1668.
He remained in private practice for the next 15 years, arguing several significant cases in the House of Lords relating to King James II exercise of power.
He married, by licence dated 1 December 1687, Anne Joliffe, daughter of John Jolliffe) of St. Martin Outwich, London and Woodcote Green, Surrey.
In 1701, when Thomas Trevor was promoted to be a judge, Northey was made Attorney General for England and Wales by King William III and was confirmed the following year on the accession of Queen Anne, when he was also knighted.
Northey was politically a mild Tory and during his time in parliament remained largely neutral, which allowed him to keep his appointments on the accession of King George I in 1714.