His grandfather had been MP for Hertfordshire and his father, a younger son, was a London merchant in the Hamburg trade.
In 1762 he succeeded to the family estates on the death of his brother, and in February 1763 returned to England, where he settled down as a country gentleman.
The English Chronicle wrote of him: “His infirm state of health prevents him from all attention to his parliamentary duty, sometimes for a whole sessions together.
This amiable character, however, in private life, has so endeared him to his constituents, that notwithstanding several gentlemen of the first opulence in the country have attempted to supplant him, and have promised a stricter attention to the duties of so important a trust; their efforts have, hitherto, proved totally nugatory ... Mr. Halsey resides mostly in the country, where his humanity and generosity, and a friendly familiar intercourse with his neighbours, have gained him the most universal esteem” He stood for Hertfordshire again in 1784 but was defeated.
[1] His estates, including that of Gaddesden Place, were inherited by Sarah, who had married Joseph Thompson Whately, later the MP for St Albans.