A native of Mexico, Martinez studied history at Yale University, graduating cum laude, and subsequently received a master's degree in Russian history from Stanford University and a law degree from Columbia University, where he served on Law Review. After law school, Martinez practiced communications law in Washington, D.C. and served as a law clerk for a federal district judge in Dallas.
Switching to journalism, Martinez worked as a reporter for The Pittsburgh Post-Gazette, where he joined the editorial board in September 1995. He also worked as a reporter for the Wall Street Journal before becoming an editorial writer at The New York Times in the summer of 2000. There, he served as assistant editorial page editor and a member of the editorial board, and it was for work done there that he was a 2004 Pulitzer Prize finalist for editorial writing. After moving to the L.A. Times in September 2004, Martinez's duties were expanded to include oversight of the op-ed page and "Sunday Current," in addition to his responsibilities for the editorial page, upon the October 2005 resignation from the paper of Michael Kinsley, who had served as editorial and opinion editor.
Martinez resigned from the Los Angeles Times in the wake of a scandal involving the selection of Hollywood producer Brian Grazer as the first in a quarterly series of "Sunday Current" guest editors. When it was publicly revealed that Martinez was romantically involved with a public relations executive who had previously represented Grazer, publisher David Hiller decided to cancel the section only three days prior to its planned publication. Interpreting this move as a sign of "no confidence" in his editorial judgment, Martinez resigned.
As a part of the scandal leading to his resignation from the L.A. Times, Martinez publicly admitted a romantic relationship with Kelly Mullens, a public relations executive. Martinez is presently separated from his wife and in the middle of divorce proceedings, but reportedly did not begin dating Mullens until after he and his wife split.