Lehr was born in Monroe, Michigan and attended St. Mary’s private school and graduated from Monroe High School in 1897. He graduated from the law department of the University of Michigan at Ann Arbor in 1900. He was admitted to the bar the same year and commenced practice in Monroe. He moved to Port Huron in 1905 and continued the practice of law there. He later returned to Monroe in 1916 and served as city attorney from 1918 to 1922 and 1928 to 1930. He was a member of the board of education of Monroe 1926 to 1936 and served as its vice president from 1930 to 1936.
In 1932, Lehr defeated seven-term Republican Earl C. Michener to elected as a Democrat from Michigan's 2nd congressional district to the 73rd United States Congress, serving from March 4, 1933 to January 3, 1935. He was defeated by Michener in 1934.
Lehr was a delegate to the 1936 Democratic National Convention and was a member of the Monroe Port Commission from 1936 to 1942. On July 2, 1936, he was appointed by President Franklin D. Roosevelt to be United States Attorney for the Eastern District of Michigan and served in that position until September 2, 1947. He resigned to devote his time as head of a fraternal beneficiary association in Detroit.
John C. Lehr died in Monroe and was interred there in St. Joseph Cemetery.