He sort of was.
Here is a statement from the University of Chicago Law School (emphasis added):
The Law School has received many media requests about Barack Obama, especially about his status as "Senior Lecturer."ML/NJFrom 1992 until his election to the U.S. Senate in 2004, Barack Obama served as a professor in the Law School. He was a Lecturer from 1992 to 1996. He was a Senior Lecturer from 1996 to 2004, during which time he taught three courses per year. Senior Lecturers are considered to be members of the Law School faculty and are regarded as professors, although not full-time or tenure-track. The title of Senior Lecturer is distinct from the title of Lecturer, which signifies adjunct status. Like Obama, each of the Law School's Senior Lecturers has high-demand careers in politics or public service, which prevent full-time teaching. Several times during his 12 years as a professor in the Law School, Obama was invited to join the faculty in a full-time tenure-track position, but he declined.
I have never heard exactly what course/s he taught as a lecturer. I would like to know that. Not all courses in law school are about the Constitution yet he is always referred to as a “Constitutional Law Professor.”
But consider that they put "professor" in the lower case.
They're saying that he taught there -- that he was considered a teacher in the law school.
That's not really anything controversial or enlightening.
A lot of people may have read the UC statement as equating a "Senior Lecturer" with a "Professor" (capital P) and that's just not the case.
Ditto with the rest of "professor of constitutional law." (again, small c, small l).
The president likes to give the impression that he taught Constitutional Law -- checks and balances, federalism, the whole ball of wax.
In fact, he didn't. His course on voting rights dealt with an aspect of constitional law, but a rather small and specialized one.
Obama wasn't a "Professor of Constitutional Law" as some people believe, and as he likes to let them believe.