Mary Test is the twin sister of Susan Test and the older sister of Johnny Test. She and Susan are geniuses capable of inventing whatever the show feels like doing each week, with Johnny often acting as their guinea pig. She and Susan both harbor a crush on Gil, who lives next door. Out of the two twins, Mary's mindset leans more toward science-fiction than proven science, but is often proven to be the voice of reason.
Mary is voiced by Brittney Wilson in the first season and by Ashleigh Ball in the second, third and fourth seasons. When Ball left the show to focus on much better things, Wilson returned as the voice of Mary for the fifth season onward.