At various points in her life Mon Mothma served as a senator, the leader of the Rebel Alliance, and Chancellor of the New Republic. She debuted in Return of the Jedi and has since appeared in numerous Star Wars productions including films, TV series, books, and comics.
Caroline Blakinston played Mon Mothma in Return of the Jedi. Genevieve O’Reilly played her in Revenge of the Sith (the scenes were deleted from the final cut), Rogue One, and she voiced Mon Mothma in a few episodes of Star Wars Rebels. Actress Kath Soucie voiced Mon Mothma in the TV series The Clone Wars.
Some of the canon books you can find Mon Mothma in include Rebel Rising, Lost Stars, Leia, Princess of Alderaan, Moving Target, and the Aftermath trilogy. Mon Mothma has also appeared in several issues of the Star Wars main comic series and several episodes of the non-canon TV series The Freemaker Adventures.
Here’s a video explaining a bit of Mon Mothma’s history from Star Wars Explained. (note that this video is from 2016 so it’s missing the more recent content, but it’s still a good overview of her life).
Check out the entire list of women in 365 Days of Star Wars Women in the Women in Star Wars Index. It includes highlights from each post plus notes which posts include new 365 interviews with actresses, writers, artists, and more.