First recorded by The Spencer Davis Group (US #10/UK #9/CAN #1 1967).
Other hit version by Chicago (US #49/UK #8/CAN #8/IRE #13 1971).
From the wiki: “‘I’m A Man’ was written by The Spencer Davis Group singer-songwriter Steve Winwood and record producer Jimmy Miller (not to be confused with the Bo Diddley song), and released as a single by The Spencer Davis Group in early 1967. It would be the last hit single by the band before the brothers Steve and Muff Winwood left the group to pursue solo careers.