The Lascelles were part of a network of mercantile interests in London, Ireland, New England and Barbados, with branches across Yorkshire, including Northallerton, Durham, Whitby, York, Harewood House and Terrington.

He had two surviving brothers, Francis (1672-1753), vicar of Knottingley, Yorkshire and William, killed at Mons in 1709, plus three sisters, Maria, Elizabeth and Sarah.

[8] The Dunkirk merchants strongly resisted the degrading of their port, adding political considerations to the complex engineering issues of eliminating access from the harbour to the sea.

Lascelles also acted as temporary Director of Engineers from 1720 to 1722 and deputy Surveyor-General of the Ordnance when Armstrong was absent; when he returned to England in 1725, he was exhausted.

At the beginning of the War of the Austrian Succession in 1740, he was asked to join the Cartagena Expedition led by Lord Cathcart; his duties in England made this impossible, while his request for the Colonelcy of a regiment in return was refused.

[10] When Armstrong died in July 1742, Lascelles succeeded him as Chief Royal Engineer, whilst also being deputy to the Lieutenant-General of the Ordnance and visited the strategic port of Ostend in 1744 to assess its defences.