According to the invention, there is provided seed and plants of the corn variety designated I900420. This invention thus relates to the plants, seeds and tissue cultures of the variety I900420, and to methods for producing a corn plant produced by crossing a corn plant of variety I900420 with itself or with another corn plant, such as a plant of another variety. This invention further relates to corn seeds and plants produced by crossing plants of variety I900420 with plants of another variety, such as another inbred line, and to crosses with related species.