Martinez attended the School of Visual Arts in the mid-1990s. She has discussed the challenges she faced as often the only female student in her cartooning classes. For much of the latter half of the 1990s, Martinez worked as a background assistant/inker for other creators, with her earliest known work being her 1993 contribution to the series Cable. Martinez counts 1999 as her big break in comics when she was working as an assistant to Marvel editor Joe Quesada. Via this connection, she became the lead artist on Iron Man, working on the book for the better part of 2000 and 2001. After a series of short stints and one-shots for the next few years, Martinez became a regular artist on the Season 2 and Season 3 Heroes webcomics produced in 2008–2009 to supplement the popular television show of the same name. Moving to DC Comics, Martinez co-created the villain Knightfall in Batgirl #10 (August 2012). In recent years, she helped create the artwork, including cover art, for a special commemorative issue of Riverdale by Archie Comics. Martinez penciled most of the first five issues of World of Wakanda, a spin-off from the Marvel Comics' Black Panther title that was primarily written by Roxane Gay. In 2017, she was a guest artist for the limited series Lazarus X+66 from Image Comics, written by Greg Rucka. She also worked on five issues of Marvel Comics' Moon Girl and Devil Dinosaur. In 2019, Martinez released the series Omni with writer Devin K. Grayson for the Humanoids Publishing comic line. In 2020, art by Martinez was included in the exhibit Women in Comics: Looking Forward, Looking Back at the Society of Illustrators in New York City. Martinez is on the faculty of the School of Visual Arts.