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Prior to his career in voice-overs, Len was a stand up comedian.
He appeared on The Tonight Show several times, on a CBS special called, The Nut House, an NBC special called The Future Lies Ahead and appeared in night clubs and theaters in Las Vegas, New York, Chicago, Los Angeles, etc. with such luminaries as Frank Sinatra, Peggy Lee, Lena Horne, Ella Fitzgerald to name but a few.
At an engagement at New York’s Copacabana, a producer for an advertising agency approached Len and asked him if he would be willing to do some of the voices in his act on a series of commercials. Len agreed. Two weeks later, when he received a check for the few hours of what was an easy and enjoyable experience, Len was shocked at the large amount of the check. He called the advertising agency, saying there had to be some mistake. They said they would look into it. Two days later a representative of the agency called back and apologized for the error and said they would send Len the rest. It was at that moment that Len decided to make voice-overs his career.
During Len’s career in voice-overs he recorded over 25,000 commercials, including the voice for "Punchy" the iconic Hawaiian Punch mascot - plus dozens of animated cartoons, and voice-overs in motion pictures.
Len won the advertising industry’s coveted CLIO award for best voice-over actor/announcer. He co-wrote and co-produced the short animated film The Crunch Bird in which he did all five voices. The film won the Academy Award for best animated short. He also directed the sound track for the film. He also provided several voices for the 1970 Christmas animated special, The Night the Animals Talked.
Len wrote some comedy with Woody Allen. He appeared with Allen on screen in Allen ’s What's Up Tiger Lily, and did the voices of most of the main characters in that film. Voice-overs in Allen’s Take the Money and Run and Sleeper followed. He and Allen remained friends until his death.
In 1964, Maxwell released the Christmas/Monster-themed LP, "A Merry Monster Christmas". This album entertained families for decades with its head-on collision of classic movie monsters with traditional Christmas observances. The album became difficult to find until Maxwell updated and re-released it on CD in 2005.