
He was born Mladen Sekulovic in Chicago in 1912, and got his start in Serbian folk theater in the 'hood. He grew up in Gary, Indiana, but returned to Chicago to audition for the Goodman School of Drama at the Art Institute of Chicago after high school. He had to work hard to rid himself of a Serbian accent!
When he went East to make it, he fell in with Elia Kazan and others at the Group Theater, and was a founding member of the Actor's Studio.
He figured prominently in my childhood for two reasons: the ubiquitous American Express ads, and re-runs of Streets of San Francisco, which played (along with Green Acres) at 4:00 am on my grandparents' huge black and white set in California when we spent Christmas there. Jet-lagged and wired, but under obligation to keep quiet for the sake of grown-ups, my brother and I spent about 10 days each December soaking up all the t.v. we weren't permitted to watch at home.
Only later did I discover his serious acting chops: the well-deserved Academy Award for his role as Mitch in Streetcar, my favorite: Fr. Pete Barry in On The Waterfront, and Gen. Bradley in "Patton," the Actor's Studio connections.
Here he is with Rosalind Russell in "Gypsy."

He was a loyal husband and a loyal friend, too. He stood by Kazan after he (Kazan) suffered reverse blacklisting for testifying against Communists before HUAC. Malden refused to turn his back on Kazan for that, causing many people to shun him, too.
It was Malden who nominated Kazan for his highly belated lifetime achievement award from the Academy.
Incidentally, one of his last roles was as a priest advisor to President Bartlett on "the West Wing." Malden told the story of bringing his stole and breviary from On the Waterfront to the set for that part --and having all the actors touch them almost as relics.
He wrote When Do I Start?: A Memoir
The LA Times obituary has a nice slideshow accompaniment.