This may help ! 
The first modern graffiti writer is widely considered to be Cornbread, a high school student from Philadelphia, who in 1967 started tagging city walls to get the attention of a girl. But it was only in the 1980s that galleries began to showcase graffiti as artwork.


‘The History of American Graffiti:’ From Subway Car to Gallery
Arts Mar 31, 2011 3:25 PM EST
Since its explosion onto city walls and subway cars in the 1970s, the increasing popularity of graffiti as an art form has won commercial success for its artists and a regular presence in pop culture and the contemporary art world.

A new book, ‘The History of American Graffiti,’ comprehensively documents the evolution of this often controversial art movement across the United States. As kids, authors Roger Gastman and Caleb Neelon tagged city walls. Today, Gastman is a gatekeeper between the underground artists who work on the street and the mainstream world of galleries; Neelon, a Harvard grad, is a graffiti artist and educator.

For “The History of American Graffiti,” they tracked down thousands of photographs, from freight trains to city streets, and conducted hundreds of interviews with graffiti artists, ranging from pioneers to the biggest stars.

Young people were the key players in shaping the contemporary graffiti movement, says Neelon. The first modern graffiti writer is widely considered to be Cornbread, a high school student from Philadelphia, who in 1967 started tagging city walls to get the attention of a girl. But it was only in the 1980s that galleries began to showcase graffiti as artwork.

Today, auctioneers and collectors shell out thousands of dollars for graffiti-style pieces. British street artist Banksy’s documentary, ‘Exit Through the Gift Shop,’ (on which Gastman was a consulting producer) was nominated for an Oscar this year. And before Marc Ecko and Shepard Fairey were household names designing clothes or Obama campaign posters, they were (and still are sometime) street artists.

But graffiti is, by definition, a defiant and public exhibition. Gastman contends that there’s an earned respect and craft to graffiti work done outside in the streets. There’s also an intrinsic subversion and vanity to an art form that defines itself by writing one’s
0331_WildStyle.jpgname over and over again on property, which doesn’t translate when it moves into a more sterile setting like a gallery.

Neelon says, however, that artists who master the craft of painting on the street can create perhaps even greater work in studio settings, where they have more time, resources and don’t have to worry about the weather (or the police). What they might lose is the volume of people who see their work on a daily basis.

Bringing graffiti from the street into the museum venue isn’t easy, Gastman says, but he’s developed a niche for doing just that. Opening on April 17 at the Museum of Contemporary Art, Los Angeles, Gastman is a curator of “Art in the Streets”, the largest American museum exhibition of graffiti and street art.” The exhibit, which runs through Aug. 8, will showcase installations by 50 graffiti and street artists.


Above photo: ‘Wild Style mural by Zephyr, Revolt, Sharp’, 1983; front: Doze, Frosty Freeze, Ken Swift; secord row: Patti Astor, Fred Brathwaite, Lady Pink; back: Lil Crazy Legs, Revolt and Sharp, directed by Charlie Ahearn, photo by Martha Cooper

Depends how far you want to go back, cave paintings, ancient Rome had it, travellers have etched their names into monuments for centuries if not millennia. Bell ringers also call their written instructions graffiti and it was written on the walls of the bell towers.

You may also enjoy watching this film, Wall Writers: Graffiti in its Innocence by Roger Gastman, and the accompanying book by the same name, which is pretty comprehensive (https://wallwritersthemovie.com/). Same author as American Graffiti which was recommended above, but more narrowly focused on the 60s-70s birth of the movement.

Another incredible source is is Graffiti Kings by Jack Stewart (https://www.amazon.com/Graffiti-Kings-York-Transit-1970s/dp/0810975262). If I'm not mistaken I believe Jack Steward did a PHD on the development of the graffiti movement, and interviewed the teenage innovators of the art form throughout the early 70s. So it's fascinating to get their perspective and firsthand accounts when these now legends were actually 14 and 15 year old kids painting trains. Although the content was in the moment, it was never widely published until much more recently in 2009.

www.beyondthestreets.com/shop/cornbread-print this is now trending at around 400 USD on ebay!

Massive rip off merchants on ebay for the unaware...many of them in France

TAKI 183

Maybe "Killroy was here" during WW2

link

"I would say that graffiti 'art' began/begins in the eye of the beholder" - WHiTEFiSH

"I'd been painting rats for three years before someone said 'that's clever it's an anagram of art' and I had to pretend I'd known that all along." - BANKSY

Funny, last week, I was at a conference made by gallery owner Magda Danysz, supporter and curator of great names of Street-art.
Of course, Wearalogy is right (Philadelphia, Cornbread, Cool Earl - first there was no crown on the tags ), he just forgot Taki183 (the 183 is the number of his streetlights ), Seen of course, the pope, for NY, Phase2, Snyder, etc ..
Keith Haring form Philadelphia too copied graffiti, very young, with guys as Fab five freddy et Futura 2000 ( they tagged in the 80 one of my jacket but I lost it ... brung it to galeries, as basquiat etc...)
In France we had Miss tic, Blek, Mesnager, Jonone but later on...

It is funny ( and I like it ! ) , to see a thread about graffiti writers as this forum is not very old school and prefers a lot stencils and street-art...

I asked some questions to Magda, kind of : " ok US graffiti writers comes legends because of the explosion in NY and they become legends bercause of USA/NY myth etc.. " , but why graffiti writers are still so hated everywhere ? Tox for UK ( remember Eine at his trial - see below )
www.theguardian.com/artanddesign/2011/jun/07/tox-graffiti-artist-criminal-damage

or in our towns French guys/crews as idfix, Maaf , Olaf etc.. that i prefer hundred times compared to " painters " making " beautiful walls, font etc ...

aboutgrafiti.blogspot.com/2013/10/a-phenomenon-called-graffiti-art.html

www.sprayplanet.com/blogs/news/a-history-of-graffiti-the-60s-and-70s

in a cave in Africa I'd say.

It was not until the 1960s that the modern graffiti subculture truly began to emerge. In Philadelphia, a graffiti writer claimed local highways with the mark ‘Bobby Beck in ‘59’ and soon after the writer CORNBREAD began writing “Cornbread Loves Cynthia” on the walls of his school, neighbourhood, and bus route. These were the first contenders for graffiti ‘kings’ and soon the movement grew. In New York City, teenagers began creating unique tags to represent their names and writing them on neighbourhood walls. One of these teenagers, TAKI 183, established graffiti as a large-scale pastime by tagging throughout the city in spots likely to be noticed by others. Many others followed in TAKI’s lead, including SUPERKOOL, who created the first graffiti masterpiece, or piece,’ by writing a larger, more stylish tag. Graffiti writers grew in prominence, technique, and scale by writing on subway trains at night. It was the surest way a writer to get their work seen across the city and many prided themselves on going ‘all cities’ by writing on trains in every subway line. A hierarchy among writers ranged from ‘toys,’ inexperienced or unskilled writers, to ‘kings,’ highly skilled and dedicated writers. In the early 1980s Keith Haring opened a store called the Pop Shop that sold products, like bags and t-shirts, which featured his graffiti. In the mid-1980s, the subculture truly spread beyond New York City, largely due to the help of tourists who started their own graffiti movements or brought information back to their hometowns. Additionally, attention from media, gallery owners, and documentary makers ensured that the movement gained lasting recognition and an international reach.
(CORNBREAD, one of the first to tag for non-gang related reasons. He gained widespread attention for daring tags such as one on a zoo elephant and another on the Jackson 5 jet)

And
1970 to 1980 London and the underground was covered by messages some of anti-war, anarchist, feminist and anti-consumerist.
wordery.com/the-writing-on-the-wall-roger-perry-9780993152009?currency=GBP>rck=QnFmMzNrKzBHYzBZZlZneDdqY2ExZVQwMzI5dTRMNVMyYWExbFFpOHJoRnYzNm1vMVRlL2RGZWsyenByd2RMYlNGRjJyQlVzWkdDdkpzK01mUFh2Y3c9PQ&gclid=EAIaIQobChMI2aa_rLiI3wIVQrTtCh1HgwRxEAYYAiABEgL-GPD_BwE#

wordery.com/medieval-graffiti-matthew-champion-9780091960414

It's an arbitrary distinction, impossible to define in any agreed way. It's not like there was 'historical' street art followed neatly by 'modern' street art after some identifiable date. There has always been street art, for as long as there's been streets in which to create it.

It's a continuum.

Whatever you point to as 'the first', someone will come up with its forerunner.

Perhaps what you're really asking is, who were the first people to create street art that is (retrospectively) perceived (mainly, it has to be said, by academia and the media and galleries, who like to consider themselves the arbiters of such things) as 'product', worthy of academic consideration or media coverage or inclusion in galleries.

Even at that I think you'll be struggling for a definitive answer.