Lehner received AB degrees in philosophy and mathematics from Harvard College and is an honors graduate of Columbia Law School.
Subsequently, he joined NRDC as director of the water program from 1994 to 1999, where he brought attention to the problem of stormwater pollution.
From 1999 to 2006, Lehner served as chief of the Environmental Protection Bureau of the New York State Attorney General's office.
He supervised all environmental litigation by and against the state and developed multi-state strategies targeting global warming and air pollution from the nation's electric utilities.
[3] Lehner also initiated watershed-wide enforcement programs and led cases addressing invasive species, wildlife protection, and public health.[which?]