Short answer: it’s rather hard to tell. 

However, though my (armchair) research cannot determine where the phrase first originated, I believe I have figured out why it’s so hard to find.

I didn’t find the phrase “Hold’er Newt!” (either on its own or followed by a subsequent phrase such as “she’s headed for the pea patch!”) printed in the newspapers I can search online earlier than 1922. This could mean that it’s a 20th century invention, however, vernacular turns-of-phrase are hard to find in official publications from over 100 years ago in general.

On an episode of the podcast A Way With Words they speculated that soldiers participating in WWI may have spread the phrase from its point of origin around the country, but this theory is unsourced. According to a woman who called in to their program, “Hold’er Newt, we’re goin’ round the corner” was a phrase regarding reckless driving used by a woman she knew from Cincinnati who was born in 1922. This woman also lived in Eastern NY State. The hosts noted that per a “dictionary of catchphrases” from 1922 “newt” was perhaps slang for dolt or idiot.

I believe that “Hold’er Newt” was 1) something one said when things were getting a little out of hand and needed to be reigned in, 2) spread around the country, obscuring its geographic origin, and 3) Newt, possibly a dumb-dumb, is the one being admonished to get a handle on “‘er.”

One evening in 2008 when I visited my late cousin Ron Losee at his home in Ennis, Montana we went outside and the screen door was about to bang closed behind us. I went to grab it, spontaneously calling out “Hold’er, Newt!.” This 93 year old man stopped in his tracks, looked at me and cried “now there’s an old Dutchess County saying I haven’t heard in a while!” He had relocated to Montana in the 1950s from the northeast and grew up in Upper Red Hook where he was born in 1919. 

Tim Sample, a humorist born in 1951, wrote about how he was familiar with the phrase from growing up in Maine in the Boothbay Register:

“Loosely translated it meant: “Just hang on awhile until we can get this situation under control.” If a gang of men was lifting a heavy load and it suddenly began to shift dangerously to one side, somebody would yell, “Hold ‘er Newt!” signaling the crew to stop and maintain a steady pressure (hold her) until help arrived.”

On searching through the newspaper archive website Old Fulton New York Post Cards, I found the phrase repeated hundreds of times in dozens of newspapers between 1922 and the 1930s and sporadically as late as 1983. In the body or in advertisements, I found it appearing mostly as a headline or tagline in the 1930s and a few times decades later. 

“Hold ‘er Newt.”—Newton P. Klemann had quite an experience last night near Olmito when his car plunged into the ditch and he found himself gliding through the air with nothing but the steering wheel in his hands. The car was not injured and after fixing the steering wheel Newt “reared” for home.  – Brownsville Texas Daily Herald, 16 Dec 1922

WHEN YOU’RE CASEY DRESSED YOU LOOK YOUR BEST… jackets, toppers, neckwear, handkerchiefs, but HOLD ‘ER NEWT, you’ve got to VISIT OUR EMPORIUM… – Penn Yan, NY Chronicle-Express 2 Aug 1951

The place it was most frequently repeated was as a line delivered in a comic called The Old Home Town by Lee Stanley. The comic (as far as my search can tell) began in 1921 and the phrase was first seen in 1922 and throughout the 1920s, and occasionally in later comics. This one-panel comic always presented a busy Richard-Scarry-esque scene from the titular Home Town where some silliness or other was befalling its residents. Often in the background one can spy a horse or mule behaving badly, rearing up and kicking, while a man with a tuft of curly hair atop his head attempts to rein it in and someone shouts at him “Hold’er Newt, she’s a rearin’!”

This was repeated in dozens of individual comics between 1922 and 1944 that I found online and was printed in newspapers in Niagara Falls, NY, Brownsville, TX, Athens, GA, and Perth Amboy, NJ and probably all over the country wherever it was syndicated. According to Stanely’s obituary in the Chicago Tribune of February 13, 1970 it was printed in over 400 publications. Newspaper readers in this time would surely have had this phrase compounded into their lexicon by The Old Home Town if it wasn’t there already.

In 1932, country recording artist (and two-time governor of Louisiana known for his recording of “You Are My Sunshine”) Jimmie Davis recorded a goofy little song called “Hold’er Newt.” The phrase “Hold’er Newt, she’s rarin’!” is called out because “that old grey mule” is misbehaving. 

The phrase and variants of it were used to advertise used cars, menswear, and in one instance for a welder in newspapers of the 40s and 50s. 

Hold ’Er Newt was also the title of a long-forgotten short-lived 1950’s children’s puppet TV show that ran opposite Howdy Doody in some northeast markets. According to Tim Hollis in his 2010 book Hi There, Boys and Girls! America’s Local Children’s TV Programs, the plot “took place in the Figg Center general store, where a group of old geezers gathered to swap whoppers with the owner, Newt Figg.” In one episode, a character called Mr. Nosegay tried to buy a cravat from a puzzled Newt who didn’t know what he was after. It would seem 1950s kids didn’t get the reference, either. It had a very brief run on ABC at 5:30 PM in 1950. The title and subject matter are lifted straight from Stanley’s The Old Home Town comic.

I could not find video or even still images of the TV show and it’s doubtful any are extant, but you can go to YouTube and listen to this goofy song. The chorus is the best part.