I had a big male tabby ( 24 lbs) that I use to walk daily on a leash with a collar though, not a harness, as he didn't like the harness at all. But he was trained when he was just 6 weeks old.
Believe me when I say that you get lots of stares from people when you are walking a 24 pound cat on a lead especially when at first glance they thought it was a small dog.