I ran into one of the ten longest running, one I wouldn’t have thought of.
Today’s Ferd’nand strip had a reference to “0,10” and I realized for the first time that it was not an American comic, so I looked it up.
The strip started in 1937, distributed by Copenhagen’s Presse-Illustrations-Bureau. It was created by Henning Dahl Mikkelsen, who had worked in animation before becoming a newspaper cartoonist. Lacking language barriers, it quickly became a hit all over Europe, and made “Mik” (as Mikkelsen called himself in signing Ferd’nand) an international celebrity. Mik brought his strip to America on November 10, 1947.
This site claims Thec Katzenjammer Kids is the oldest strip still in syndication:
Ally Sloper is arguably the oldest comic strip, little recognized in the US although he predated The Yellow Kid 9often cited as the first comic strip) by some twenty years.
But Ally hasn’t run continuously, ending his first run in 1916. He’s had a few disconnected runs over the years, the most recent in the 1970s. He shows up in a cameo (not surprisingly) in the second “volume” of The League of Extraordinary Gentlemen.
The original Pogo strip began in 1948 and ended in 1975, a couple of years after creator Walt Kelly died. It was revived for a few years in the early '90s, and the new version (with the involvement of Kelly’s son) was actually pretty decent, but it naturally lacked the magic of the original. There is no version currently running.