To rotate a point \((x, y)\) counterclockwise by \(270^\circ\) about the origin, you can use the following transformation:
\[ (x, y) \rightarrow (y, -x) \]
For the point \(A(1, 2)\):
- Identify the coordinates: \(x = 1\), \(y = 2\).
- Apply the rotation transformation:
\[ A' = (y, -x) = (2, -1) \]
Thus, the ordered pair for \(A'\) after a \(270^\circ\) counterclockwise rotation about the origin is \(\boxed{(2, -1)}\).