Currently the longest single-state mileage of any US route is 893 miles (US 83 in Texas). Historically, before US 101 was truncated to Los Angeles, that route was undoubtedly the record-holder: the California segment of US 101 was about 941 miles (give or take a few, depending on which year, since the alignment changed many times and in many places).
Historic US 99 in California was the second-longest route through a single state, although its mileage is even more difficult to measure, since it was replaced by I-5 in many areas, and as a result some segments of the original route have been vacated. The Historic Highway 99 Association of California offers a length of 917 miles. Although that page does not specify a year, I am sure that figure is reliable, since my own back-of-the-envelope calculation yielded a similar distance. (Note that drivers choosing US 99E between Sacramento and Red Bluff had to travel 2.2 miles further than those who chose US 99W.)
Thanks to this blog post, I recently became aware of "the Palm and the Pine" along what is now CA state hwy. 99, just southeast of Madera. In the median of the highway, a palm tree and a pine tree are planted side by side (one representing southern California, and the other representing northern California):
It depends how you measure... and specifically what you measure. If you are familiar with the mileage signs posted at the terminus points of US 6 and US 20, you might point to those and say, "The answer is plain to see: US 20 is 160 miles longer than US 6."
If both of those signs were accurate, then obviously US 20 would be longer. But the fact is, only one of those signs is accurate, whereas the other significantly overstates the true mileage. I make that statement based on measurements I first took in 2016, and later verified in 2020, using a method that I believe to be quite accurate (and in some cases, more accurate than the "official" distances posted by state departments of transportation). So let's take a look at those mileages:
US 50 is one of the longest highways in the U.S. How long is it? Well, there's actually some debate on that (this page has more details about why that is).
Both the Maryland and California state departments of transportation have placed reciprocal mileage signs at the terminus points, each claiming the total distance is 3073 miles. This photo was taken in Ocean City MD, looking along the east beginning of US 50...
(last updated 8/21/2020)
"One great red line"
For about a quarter-century, US Route 6 was the longest highway in the country (and it may still be, depending how one measures). In Jack Kerouac's novel On the Road, the main character described it as "one great red line across America". During those years it ran between Provincetown MA and Long Beach CA, and I believe there has never been a longer highway in the United States. Exactly how long was it? Well, believe it or not, the answer to that question is debatable (see this page for more details about why it is difficult to calculate).