[2] Karlan earned his bachelor's degree in business administration with an emphasis in accounting from the University of Colorado at Boulder in 1989, where he was valedictorian of his graduating class.

[5][6] Karlan started his legal career by clerking for Judge Julian Jacobs of the United States Tax Court from 1993 to 1995.

[5][6] In 1998, he joined the Office of Chief Counsel of the Internal Revenue Service (IRS) as an attorney, and served there until 1999.

[8] On December 31, 2006, when he was simultaneously hosting New Year's Eve Galas at both the French embassy and the Washington Plaza Hotel, The Washington Post wrote a cover story about Karlan that characterized him as "an expert party guy.

[12] In February 2011, Karlan hosted the nation's first-ever speed dating at an auto show, at which Washington Post columnist Carolyn Hax provided advice on dating and then the speed daters went from car to car in the General Motors exhibit at the Washington Auto Show participating in a different date in each car.