Mackey is a street kid from Gotham.
Mackey is one of the street kids that were attacked by Patti and Doug, subordinates of Dollmaker, pretending they were part of the Mayor's Homeless Outreach Program. Mackey was chased by Doug, though he escaped by crashing through a window of a restaurant. He was later interrogated by James Gordon and Harvey Bullock at the GCPD Headquarters, the latter of whom was under the impression he was lying, threatened to harm Mackey unless he told them the truth.
Mackey was later brought in for questioning about his friend Cat by Bullock. Although initially reluctant, he eventually (after being bribed $200 by Alfred Pennyworth, who was helping Bullock at the time) told him to ask Fish Mooney where to find Cat's fence.
Despite an unquestionably tough life, Mackey seems to retain a fairly happy-go-lucky approach.