Klempner grew up in New York City, and attended Cornell University, graduating Phi Beta Kappa in 1997, and winning a J. William Fulbright Fellowship.
Klempner spent nearly a decade talking with and getting to know Dutch Righteous Among the Nations in order to write The Heart Has Reasons: Holocaust Rescuers and Their Stories of Courage, Cleveland: The Pilgrim Press, 2006.
Because Klempner is the son of a Jewish immigrant who barely escaped the Holocaust, he found that his subjects sometimes looked at him as the child or grandchild of the Jews they rescued.
In Spring of 2012, Klempner addressed members of the United States Congress and their Staffs on the occasion of Holocaust Remembrance Day (Yom HaShoah).
Klempner's articles have appeared on the internet at such sites as Common Dreams, Alternet, The Huffington Post, The Social Edge and Sojo Mail, the weekly newsletter of Jim Wallis' Sojourners community.