Reform on Monday announced that Sutton Reddicap councillor Jex Parkin, 24, had been elected as its group leader on the council, with Charles Latchford as his deputy.
The Green Party took 19 seats, with Labour on 17 and the Conservatives with 16.
The Liberal Democrats have 12 seats, alongside 13 independents.
Glebe Farm and Tile Cross was the last of 69 wards left to be called after teams ran out of time to finish counting votes on Friday evening, with counting restarting from 13:00 BST on Monday.
Talks will now begin between parties on forming a viable administration. No group has even a quarter of the seats on the authority, let alone getting above the half (effectively 52 seats) required for a majority.