The Tootsie Pop commercial with the kid asking the animals how to get to the Tootsie Roll center of a Tootsie Pop (you know which one I mean) has been airing nationwide pretty much unedited since its first air date, which begs the question: how old is it, and does this qualify it as the single longest-running ad ever?
The famous Life Cereal commercial with Mikey saird for years in the late 1970s and early 1980s.
As a related question, what advertising campaign has been going on the longest.
For example, the Marlboro Man campaign has lasted approximately forever. When did it start, and are there any longer campaigns out there.
“A diamond is forever” has had a long run.
The Juicy Fruit commercial with the people water-skiing ran for years and years. I think it debuted about 84 or so and ran well into the 90s.
Chevrolet’s “Like a rock” commercials have to be the longest campaign. I can’t find a cite to back this up, but I remember hearing (and hating instantly) that song in a Chevy commercial in either '88 or '89.
I would imagine a locally produced commercial would be the winner for most repeated single commercial. Does K-SHE 95 in St. Louis still air that commercial where the Mom is dancing to Brown Sugar? “Dad, she’s doing it again!”
I don’t know how widespread they are, but there’s a tire store in Washington state with a commercial that says, “If you’re ever unsatisfied with one of our tires, please feel free to return it.” On screen, an older woman is shown throwing one of their tires through the store window and shattering the glass. I don’t know when it first aired, but it’s been running off and on for at least the last ten years. From the video quality, I would guess it was shot in the '70s or early '80s.
According to The Ad Council, the longest running ad in their history is Forest Fire Prevention – Smokey Bear (1944 - Present)
Does APEX technical school count?
How about that Johnsonville Brat ad? It has to have been running since the 1970’s.
If you live within broadcast range of Chicago, Illinois, it has to be the Empire Carpet commercial “488-2300. Empiiiiire!” That guy just won’t die. The commercial isn’t exactly the same as it was 20 years ago, but close.
Not really a contender for the title, but that group of Mentos commercials with the same song ran for a very long time.
Oops, it’s actually “588-2300” 
That would be Discount Tire Co., a tire store chain in the western US. I just saw the commercial on television today in Phoenix, and I distinctly recall the same ad being on TV when I was a little kid in the early 80s.
I mentioned it to my father recently, and noted how it seemed to have been aired forever. He said he recalled it already being on when he moved to Arizona in the mid-70s, so it’s possible those cheapos have been airing the same commercial continuously for as long as 30 years.
(For what it’s worth, the mountain the background is Moon Mountain in Phoenix.)
I forgot … in Kansas City, there’s been a trend of running “retro commercials,” original color commercials of the advertisers from the late 1950s and early 1960s. An ancient Hy-Vee Supermarket commercial includes a very 1950s sounding jingle (“Where there’s a helpful smile … in every aisle!”), sung in 1950s vocalist style by a woman, with the hard-to-describe bing-like notes common in the day. Elderly Standard Home Improvement commercials also air frequently. The commercial has a modern voiceover for a few seconds, where the original six digit phone number is displayed.
Are retro commercials airing in other cities?
Every Christmas, for about 40 years (maybe longer), in Oklahoma City, BC Clarke Jewellers runs the same song as a commercial for their anniversary sale. The TV ads have evolved, but the radio is EXACTLY the same as when I was a child. My mom says it goes back further than that even.
"Jewellery is the gift to give
'Cause it’s the gift that’ll live and live
So give the gift you know can’t fail
From BC Clarke’s
Anniversary sale…"
Are you guys talking about series of commercials (i.e. the “Like a Rock” Chevy commercials still use the same slogan and song, but they’re new commercials), or are you talking about running the actual original version? The tootsie pop commercial is the original, grainy, simple, animated commercial, not just a long-running campaign.
So is the BC Clarke radio ad.
From Christmas 1956 til now and beyond.
You can listen to it, too. 
How about the VB (Victoria Bitter beer) ad campaign that has been running since the mid-60’s with John Mellions voice.
It is still running now as a version of the same commercial, still with John Mellions voice. The copy changes every few years. I dont know how that works, since he died in the late 80’s. The magic of television.
Hurling tire commercial a retread for 28 years
The claim that it’s the longest-running commercial ever seems to hinge on the fact that it’s exactly the same now as it was when first aired, not just the same campaign or with new copy as in the Victoria Bitter spots and others.