1510s, "benevolent goblin supposed to haunt old farmhouses in Scotland," diminutive of brown "a wee brown man" (see brown (adj.)).
The brownie was believed to be very useful to the family, particularly if treated well by them, and to the servants, for whom while they slept he was wont to do many pieces of drudgery. In appearance the brownie was said to be meager, shaggy, and wild. [Century Dictionary]
As "small square of rich chocolate cake," often with nuts, 1897. As a brand-name of a type of inexpensive camera, 1900. The name for the junior branch of the Girl Guides or Girl Scouts is by 1916, in reference to their uniform color. Brownie point "notional credit for an achievement; favour in the eyes of another, esp. gained by sycophantic or servile behaviour" [OED] is by 1959, sometimes associated with Brownie in the Scouting sense but is perhaps rather from brown-nose.