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):
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...
Many Route 66 tourists simply enjoy the romance and the lore of the fabled road. The facts aren't all that important, and if one of the legends turns out to be a tall-tale, they'd rather not know about it. And that's fine -- if you're one of those people, I say you gotta do you -- get out there and enjoy the Mother Road.
...then you might want to stop reading here, because this page contains research that discredits some of the "conventional wisdom" about US 66 that is found elsewhere. On the other hand, if you're interested in facts and details, then please carry on.
Why is it difficult to obtain accurate end-to-end mileage for many US highways?
This comes as a surprise to some, but US highways are actually not federal highways... at least not in the sense that they are owned and maintained by the federal government. Rather, the US routes are actually just state highways... although they are "special" in the sense that at some point they were granted permission by AASHO (later AASHTO) to be signposted with a US route shield.
(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).