Mark J. Penn (born January 15, 1954), is the worldwide CEO of the public relations firm Burson-Marsteller and president of the polling firm Penn, Schoen and Berland Associates. In September 2007, he released a book titled Microtrends: The Small Forces Behind Tomorrow's Big Changes, which examines small trends sweeping the world. Penn's clients have included political and business leaders, including U.S. President Bill Clinton, U.K. Prime Minister Tony Blair, and Microsoft Chairman Bill Gates; he also served as chief strategist and pollster to Hillary Clinton in her 2008 presidential campaign. Penn is married to Nancy Jacobson, a professional fundraiser.

Penn was born in New York and raised in Riverdale. His father was a Lithuanian immigrant who died when Penn was 10. He was raised by his mother Blanche, who worked as a school teacher. Both of his brothers credit Penn with keeping the family together after their father’s death. Penn graduated from the Horace Mann School in New York City in 1972. He conducted his first poll, which determined that the Horace Mann faculty was more liberal than was the country at large on the issue of civil rights, when he was 13. Penn founded Penn, Schoen and Berland (PSB) with Doug Schoen in 1975 while they were undergraduates at Harvard.

In the fall of 1976, while Penn was a first-year law student at Columbia University, he and his business partner Douglas Schoen became the pollsters for congressman Ed Koch's second (and first successful run) for the mayoralty of the City of New York. In 1977, with the campaign against Mario Cuomo for the Democratic nomination in full swing, Penn sought a way to conduct polls more quickly than the mainframe and punchcard system he and Schoen were making use of at Columbia University. He purchased a self-assembled “microcomputer” kit and created a program that could compile polls in a fraction of the time than had been done before. By creating this "overnight poll" system, Penn allowed the campaign to conduct polls to determine messages and evaluate tactics on a daily basis, a strategic advantage that contributed to Koch’s eventual victory over Cuomo.