Writer and comics expert/historian Mark Evanier is reporting that with over 19,000 daily and Sunday comic strips over 53 years, Russell Myers holds the record for most comic strips drawn by one person, without assistance, for his comic strip Broom Hilda.
There are much longer-running comic strips, like Blondie and Gasoline Alley, but these are legacy comic strips, drawn by various cartoonists who take over the strip once the previous author retires or dies. What sets Broom Hilda apart is that he's done it all himself, with no successors.
I would have thought Charles Schulz would hold the record because he famously insisted on sole authorship of Peanuts throughout its 50 year run. But Evanier says that Schulz "only" drew 17,897 published Peanuts strips.
You can enjoy Broom Hilda here, but, alas, the archives only go back to 2001, so you only have 21 years of comics to peruse.