During his long service (1779 – 1805) in India, Sandys saw action in the 1^st Maratha War (1778-1782) which was fought between the British East India Company and the Maratha Empire which, at the time, dominated almost two thirds of India. 
He fought in the 3^rd Mysore War (1790-1792) which was conducted between the Company and the Mysore Empire of south-western India. 

Appointed a Major in the 5^th Bengal Native Infantry (1803) and promoted to Lieutenant-Colonel in the 15^th Bengal Native Infantry (1804) he retired to Lanarth, near St. Keverne the following year.

The Robert Home painting (below left) shows Sandys with his first wife Charlotte, who died in 1802, and his sons and his sons William (born 1795) and Allan (born 1799).

Sandys remarried and also had an illegitimate son, John Francis (born 1788) by an Indian mistress.

Sandy’s memorial in St. Keverne Church states

‘Sacred to the Memory of Lieut. Colonel Sandys late of the Honourable East India Company's Service on the Bengal Establishment, and one of his Majesty's Justice of the Peace for the County of Cornwall, who departed this life 21^st of August, 1829 in his 70th year, and whose remains are interred in a vault at the west end of this Church.
As a soldier he was conspicuous for intrepidity, particularly in the Mysore War of 1792, under the command of Lord Cornwallis, and was rewarded with high offices on the Staff of that distinguished nobleman when Governor General and Commander in Chief in India.
In 1802, on his return to England he settled on his paternal estate at Lanarth in this parish. His integrity, as a Magistrate, his unaffected piety, and extensive benevolence gained for him the esteem and regard of all around.