Sandby was born in Nottingham, and baptised there in 1731, although his date of birth has traditionally been given as 1725. In 1745 he moved to London where he followed his brother Thomas in obtaining an appointment in the military drawing department at the
While undertaking this commission, which included preparing designs for new bridges and fortifications, he began producing watercolour landscapes documenting the changes in Scotland since the rebellion, and making sketches of Scottish events such as the hanging in Edinburgh of soldier-turned-forger John Young in 1751.
He left his post with the survey in 1751, and spent some time living with his brother, who had been appointed Deputy Ranger of
He also etched a large number of plates after his own drawings, a hundred of which (including views of Edinburgh, which were published in a volume in 1765. In 1760 he issued twelve etchings of The Cries of London. He also made many plates after other artists, including his brother. In 1753–4 he published, anonymously, several single caricatures satirising
It is not recorded how long Sandby lived with his brother at Windsor, but he is said to have spent part of each year in London, and much of his time was probably spent on sketching excursions. On 3 May 1757 he married Anne Stogden, and by 1760 he was settled in London.
In 1760 he contributed to the first exhibition of the Society of Artists. He exhibited regularly with the society until the foundation of the Royal Academy eight years later, and was one of its first directors when it was incorporated in 1765. In 1768, he was appointed chief drawing master to the
Sandby made extensive journeys around Britain and Ireland, sketching scenery and ancient monuments. He made his first recorded visit to
He died at his house in Paddington on 7 November 1809, and was buried in the burial ground of