[2] He grew to fame and fortune in the city as a merchant, working with his younger brother John who ran the family shop, and eventually becoming one of Edinburgh's richest men.

He was notoriously thrifty and is credited with inventing the phrase "waste not, want not",[3] though Henry Erskine, alluding to Gillespie's snuff-fortune, said of his handsome carriage, "Who would have thought it, that noses had bought it".

He died on 8 April 1797 and is buried in a large enclosed vault on the north side of the church in Colinton Churchyard, near his house.

A quarter of Gillespie's bequest (£2,700) was left for the establishment of a free school for the education of poor boys.

The school was to educate boys between the ages of six and twelve who were required to produce proof of their poverty and a certificate of good health.

In 2020 growing awareness of Gillespie's links to the Virginian slave trade[6] caused a public petition to be raised to rename the school.