Malina Weissman was in Supergirl for 2 seasons : Season 1 and Season 2, though only in a few episodes overall.

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Season Role Notes
Season 1 Young Kara Zor-El / Young Kara Danvers Multiple episodes listed.
Season 2 Young Kara Danvers / archive footage Appeared in at least 1 episode.
So the short answer is: **2 seasons**.