Mr Kir currently serves in the Brussels Regional Government as a Secretary of State, but he will now resign to take up the job of burgomaster of Sint-Joost.
Mr Kir's socialist party fought Sunday's election with a Mayor's List. The list lost 5%, but with 44% of the vote remains by far the biggest party in the borough.
Ecolo/Groen were second with 18%.
The Mayor's List will now join forces with the Christian democrats to form the council majority in Sint-Joost.
The outgoing mayor, Jean Demannez, who belongs to Mr Kir's party, was going to stay on for a further three years, but because Mr Kir polled significantly more votes the latter demanded that the switch take place immediately.
Jean Demannez was clearly devastated by this development and called Mr Kir "a liar" because he failed to stick with the deal that he had agreed.