This question really needs to be answered at two levels - the indirect (financial and industrial) contribution, and the direct military contribution of the US.

To take the first point first:

By virtue of naval supremacy in the Atlantic, the Entente had ensured that it had virtually exclusive access to US markets for war material and by extension, finance.

This was an absolutely crucial factor in winning the war, particularly as Germany had occupied the industrial heartlands of France and deprived it of much of its coal and steel production. American credit was also crucial in financing the war, which was proving ruinously expensive. The cost of a single preliminary bombardment being equal to the cost of the latest super-Dreadnaughts, in itself enough to threaten a secondary power with bankruptcy. Over the course of the war, Britain went from being a net creditor state to a net debtor.

Had the US simply refused to provide material or credit, there is a possibility that the Entente may not have been able to continue the war.

Against this we do need to recognise that this was not being done out of any sense of altruism – goods were bought and paid for, and credit was to be paid back with interest (the UK paid off its Great War debts in 2014).

We also need to recognise that the material provided was mainly raw, the finished items being provided by the Entente. Hence when the US did finally enter the war, it did so with French and British aircraft, tanks, and guns.

As to the actual US contribution to the war effort other than bought-and-paid-for material;

The US declared war in April 1917.

One year later there were 430,000 American troops in country. But there is a big difference between simply having men in country, and actually having a trained and proficient army in the line.

By the start of the German Spring Offensives in 1918, there were 5 American Divisions active. These were either put into reserve attached to the French, or took over quiet sectors enabling the French to give some assistance to the BEF.

Between March and April, approximately 3,000 US troops were in action across the Somme and Lys - effectively the equivalent of British brigade.

Britain and the US shared a common problem in that they both had small armies when they went to war, and had to rapidly expand them.

The US had the advantage of coalition partners it could learn from, but Pershing viewed his partners as weakened by years of trench warfare and was reluctant to learn from them.

According to Pershing, it was the infantryman with his rifle and aggressive spirit that would "drive the enemy from his trenches, and by the same tactics, defeat him in the open".

In the words of Liddel-Hart "He thought he was spreading a new gospel of faith, when in fact it was an old faith exploded" The men of the AEF would pay a high price for their tactical naivety.

One German source remarked

"The American infantry is very unskillful in the attack. It attacks in thick columns in numerous waves echeloned in depth, preceded by tanks. This sort of attack offer excellent objectives for the fire of our artillery, infantry, and machine guns."

Captain Walter Rainsford observed that

"Many an officer was entirely capable of waving a vague finger over a valley marked on a map, while stating that the troops in question were 'on that hill' and, if pressed to be more precise, would give their coordinates which represented a point neither in the valley he had indicated nor the hill on which they were"

As the British had found, it is one thing to train a man to 'sort of' infantry, but its a lot more difficult to achieve a competent officer corps, or a cadre of experienced NCO's. And the price to gain these assets is paid in blood.

Muese-Argonne was the US's first major offensive and parallels can be drawn with the British experience at the Somme in 1916.

The offensive bogged down quickly with the French making much faster progress than the AEF.

The first large scale offensive by a green army that had yet to learn its craft and would pay the price in blood. The AEF was making elementary mistakes like failing to mop up after an advance, and finding they were receiving fire from their rear. This is exactly the kind of mistakes the BEF learned to avoid during the Somme, and as with the BEF and the Somme, had the war gone on another year, the AEF would have shouldered the brunt of the offensive action, just as the BEF did in 1917.

Cooke, in Pershing and His Generals, states that the AEF was "dangerously close to played out" by the armistice.

Muese-Argonne was a significant part of the 100 days offensive, but it was not the decisive battle, being one amongst many.

By October, the Belgians had renewed their offensive and were making steady progress. The BEF's 4th Army was over the Selle, and 2nd Army was over the Lys, with 5th and 3rd Armies also making progress.

On the 4th of November, 17 British and 11 French Divisions smashed through the Germans at the Sambre effectively denying the Germans the chance to re-form a new defensive line on the Meuse, after which they would advance rapidly by up to 5 miles a day.

Finally its worth pointing out the morale contribution that direct US involvement made. Germany now had to contend with a new enemy with a fresh and untapped pool of manpower that was far larger than it could hope to match. The calculus was obvious.

However, with their army unable to hold any defensive line, and Germany itself starving and collapsing into mutiny and revolution even without US involvement, there is a realistic probability that Germany would collapsed over the winter of 1918-1919 anyway.

The final arithmetic of the 100-days offensive it telling. Examining the number of prisoners and guns the each major belligerent took we see:

BEF - 188,700 prisoners and 2,840 guns

AEF - 43,000 prisoners and 1,421 guns

French & Belgians - 153,700 and 2,349 guns

I think those stats speak for themselves.

TL;DR - The direct American military contribution, while not insignificant, was not the decisive factor in ending the war. The chief American contribution to victory being in morale and materiel rather than military factors.

Sources

Peter Hart - The Last Battle

John Terraine - The Smoke and the Fire

War Office - Statistics of the Military Effort of the British Empire during the Great War