Acted twice on stage with his wife Eleonora von Mendelssohn, once in Berlin when he was performing in "The Tribune" and again on Broadway in "The Madwoman of Chaillot".
Although German-born Kosleck specialized in playing Nazis (he was once admiringly described by a film critic as "the definitive Nazi swine"), in reality his vehement opposition to Hitler and Naziism landed him on a Gestapo list of "undesirables" and he fled Germany one step ahead of a Gestapo death squad.
During the World War II era many German actors fled the Nazi regime and emigrated to Hollywood, where they often played evil SS officers or murderous Nazi spies. Some of them came to resent being typecast that way, but Martin Kosleck had no such compunctions. An outspoken anti-Nazi in the German film industry, his activities earned him the enmity of Josef Goebbels, who became the Nazi Propaganda Minister. When the Nazis finally took over Germany, an arrest warrant was issued for Kosleck. He learned that he had already been tried in absentia and sentenced to death, and he escaped Germany as an SS hit team was tracking him down. The experience only deepened his hatred of Hitler and Naziism, and he once told an interviewer that his playing Nazi killers and exposing people to the evils of Naziism was his small way of paying back what the Nazis had done to him and his country.
Later became an exhibited painter in Los Angeles and New York. Marlene Dietrich and Bette Davis both owned some of his portraits.
Was the fourth husband of Eleonora von Mendelssohn. They both fled (not together) the Nazi regime.