Well for starters, the United States has not been the world's "most dominant" power for 75 years. It was in competition with the USSR after the Second World War. There's an argument that the U.S might have been the stronger of the two powers, but it certainly did not "dominate" - which was pretty the basis for Cold War competition between the two powers.
What was naively referred to as "the end of history" or a period of global U.S primacy really only stretched from 1991 to 2007. (The dissolution of the Soviet Union to the Lehman's brother collapse).
This depends on how far we are willing to stretch the definitions. We could say Rome was the longest surviving "world" power. But in reality the Roman Empire went through a few incarnations, and by today's standards was really only a regional hegemon. It never truly possessed what could be described as a 'world power' status. I mean, the Romans never even kept a foothold above the Rhine. There were other Empires in existence that the Romans didn't really have much awareness of such as China, whom it could never hope to dominate.
The only real candidate here is the British Empire during the period mentioned. But then, we run into semantic difficulties with the term "dominate". The British Empire was powerful militarily, but it used its economic power rather than its military to protect its vital interests. The British still had a number of near peer competitors militarily, and it was not able to unilaterally impose its will on other nations through the use of either lever (see the Crimean War).
So I would say the British Empire, but it's murky.