what was the first year seat belt was standard on cars.
hmm … I’m guess somewhere around 1965 … but not certain.
IIRC, front lap belts became mandatory in 1964, and rear lap belts were mandated as of 1966.
Shoulder harnesses for the driver & front seat passenger didn’t become mandatory until…probably…1971, but I think that it took quite a few years…perhaps until the early '80s…before rear seat shoulder harnesses became mandatory.
I used to own one of these and it had factory belts in it so that covers the discussion back to that year anyway… 
http://www.old-corvette.com/1959-corvette.php
This is fuzzy ancient history but I think the '58 models came with anchors but no belts. The belts were an option at the dealer. Vettes before '58 did not have factory belts or anchors but could be fitted with belts by the dealer.
My dad bought a new Studebaker Lark in 1963 and the front seat belts were standard equipment. There were anchors for the rear seat belts, but the seat belts for the back seat were a dealer installed option. I bought a 1968 AMC Javelin and it was equipped with shoulder harnesses in the front seat as I remember.
I think that Nash offered a seat belt for the front passenger side in 1950, but it was advertised for the comfort of the passenger.
We restored a 64 Grand Prix and the original owner stated he ordered it with optional seat belts. Our family owned a 68 Buick Wildcat and it had shoulder belts that were retained in clips along the interior roof rail at the headliner when not used.
I think it depended on what model you had and how deluxe it was. Our 57 Ford Fairlane 500 had them but it was a top of the line model. Our 58 and 61 Chevy Biscaynes did not have them, but our 61 Merc. did. My 59 Pontiac did not have them and I had to add them when I bought it. Our 66 Plymouth had the lap belts but the shoulder harness was the first I saw in our 68 Dodge. I think everything after 68 had both the lap and shoulder belts.
I think back in the 56-57-58 era, Ford started to sell safety with their padded dash, recessed steering wheel, seat belts, etc. Before that and maybe for GM, they were afraid to talk safety thinking people would consider cars unsafe-duh.
Some cars had seat belts as options, some as standard equipment in the late '50’s and early '60’s. I had a '61 Mercury Montery Convertible, no seat belts. Then a new '67 Mustang which came with seat belts as standard equipment. I don’t remember seat belts in my dad’s '65 T’bird, but my brothers '65 Mustang had them.
The question really is when were seat belts required in all cars as a government safety mandate? On that point I’m not sure, but '66 to '68 is my guess. By 1970 seat belt (lap belts only) were required in all cars sold in the US.
The early seat belts had no retractors and were simply belts and buckles Like airplane seat belts still are. Many owners removed them because they were “messy” and a bother to dig out and put on every time. Few cars had “bucket” seats back then so the seat belts got pushed down in between the bottom bench seat cushion and the seat back and were quickly ignored and forgotten.
My father’s '63 Plymouth did not have seat belts as it came from the factory, but because the local Chevron stations were running a low-cost program of installing front lap belts, he was able to have them installed at a fairly low cost. The “mountings” were installed at the factory, but the seat belts were not.
As Uncle Turbo pointed out, the early belts had no retractors, and after careless drivers and passengers had allowed belts to dangle into the gutter a few times, some folks opted to stop using the grimy belts.
I had an aunt who became somewhat irrational in her later years, and for some reason she was very annoyed by the seat belts in her Chevy Malibu (circa late '70s). As a result, she took a knife and cut the belts out of the car. When I asked her why she did that, her answer was, “If I was in an accident and my hands were injured, I would be trapped in the car when I was unable to unlatch the seat belt!”
I then pointed out that those same injured hands would also prevent her from opening her door, thus trapping her just as effectively as those seat belts. The slow look of recognition on her face indicated to me that she realized I was correct, but her stubborn personality would not allow her to admit it.
Nationwide I believe was 1964 front with the rear in 1966. In the sixties I was a JC member and one of our guys had a 1962 Ford with front seat belts. They were an option at that time. He was a safety freak and bought the Ford because it had the “Deep Dish” steering wheel.
My 1965 Dodge Dart had them in the front as standard but I paid extra for the rear and the “Deluxe” version, which had retracting rollers. I wanted the rear one to secure our kids car seats
In other countries they came much later. A colleague who worked in Venezuela had a 1994 Mazda (built in Colombia) with only front belts. When they became mandatory in Europe in the 70s I remember getting a set from a wreck here and sending it to a relative there.
@Bing–you are correct about Ford. The company began pushing safety and offered, as optional equipment, seat belts and padded dashboard and sun visors as options, and all its cars came with redesigned door locks that were more resistant to popping open in a crash and the deeply recessed steering wheel hub. Unfortunately, advertising safe cars in those days didn’t pay off and people rushed down to the Chevrolet and Plymouth dealers who were pushing horsepower. Chevrolets and Plymouths were perceived to be better cars because they didn’t get into accidents. Ford backed off the safety advertising and bragged about its powerful Thunderbird V-8 engines. Sales immediately picked up.
I dunno what year it was, but my father had an ugly green Oldsmobile(I think) and it had lap belts on it, and the shoulder belts latched into the lap belts, separately, before you secured the belt across your person.
So I guess the next logical question to ask would be when the “modern” 3-point harness became standard equipment.
My 74 Olds had the modern type retracting lap and shoulder belts so must have been between 68 and 74. I had a 72 Lincoln but can’t for the life of me remember what it had.