Well as has been noted earlier, it's actually not that difficult with Mitt Romney nor even George Romney.
An interesting question would be, who would be the earliest conceivable Mormon President?
If we're looking at prominent Mormons who ran for the Presidency, I can only really recall the two Romneys, Huntsman, Udall, and, well, interestingly, Joseph Smith, Jr. himself, who ran in 1844 with Sidney Rigdon, the First Counselor in the First Presidency of the LDS Church, as his running mate. Indeed, when Smith was killed the same year, most of the Quorum of Twelve were out of Nauvoo, campaigning for him.
I think it's fair to say that Joseph Smith, Jr. didn't have a snowball's chance in the Telestial Kingdom to go that far, so let's expand our criteria, shall we?
Prominent Mormons who have served in high-profile public office. Well, there's Brigham Young who was the territorial governor of Utah, but he definitely had far less of a chance than Joseph Smith, since he openly practiced polygamy, managed to make himself hated by both James Buchanan and Abraham Lincoln, had preached that slavery was ordained by God from the pulpit, and during the American Civil War had explored the idea of unilaterally declaring Utah an independent state (not with a mind to join the Confederacy, but to set up an sovereign theocracy of Deseret).
So he's out.
Moving into the twentieth century, we have Reed Smoot, the first member of the Quorum of the Twelve not to be a polygamist since the 1840s, who in 1902 was elected by the Utah legislature to the US Senate. He was very controversial at the time, and there were these very long hearings in the Senate as to whether or not he secretly supported polygamy and whole bunch of stuff when they were deciding to seat him. They kept calling in all sorts of witnesses, asking all sorts of question, among other people called in to give testimony were the Prophet-President of the Church at the time, Joseph F. Smith, nephew of the Church's founder, and the Senators grilled him about how precisely it went about when he received revelations from God. Now you might well think that Mormonism is a very bizarre religion (I certainly do), and the very pointed questions the senators asked and the at times outlandish, at times evasive answers given make for some truly fascinating reading, they do in retrospect seem quite irrelevant to the matter at hand they were supposed to be investigating. In the end, the Senate narrowly failed to gather together the two-thirds majority needed to prevent Smoot from being seated, and he was duly sworn in.
Despite the controversy surrounding his arrival in the Senate, as he kept getting re-elected and gained seniority, he actually over time became a fairly respected politician and lawmaker, as fellow Senators and Congressmen learned that if they looked past his religion, he was most decidedly a man you could do business with. His name appears as one of the authors of the infamous Smoot-Hawley Tariff Act, which had been blamed by many as having deepened the Great Depression something severely. In 1932, despite him being an Apostle in the religious hierarchy of his Church, Utah Mormons overwhelmingly voted against him in that year's senatorial election, instead picking Democrat Elbert D. Thomas, who would go on to serve for 18 years.
Still, I would very much be sceptical that even during his best days during the Roaring Twenties did Reed Smoot ever have a chance if he had sought the Presidency (if indeed he ever wanted it, which is in itself highly doubtful). So he's out.
The next person on the list would be Ezra Taft Benson, who served as Dwight Eisenhower's Secretary of Agriculture.
Now Ezra Taft Benson was very much a staunch conservative and anti-Communist, and he seldom hesitated to mix religion and politics. In his mind, Communism was a "secret combination" prophesied by the Book of Mormon, and when he became President of the LDS Church in the 1980s, many of his sermons talked about an imminent time when the Constitution would hang by a thread, and Mormons would have to save the country, and anarchy and lawlessness was at the door, it was all very apocalyptic, all very millennialist. He was also very much a racist and an opponent of civil rights.
Add to it that he wasn't particularly popular with farmers during his time as Secretary of Agriculture, and it's quite clear that this man was never really going to have the support of more than perhaps the Utah chapter of the John Birch Society if he decided to run. Or if indeed he ever decided to seek any elected public office.
So, who else are there? I honestly cannot think of any other Mormon who was ever a high-profile name in American politics prior to George Romney.
There is something odd about the fact that the first Mormon to actually seek the Presidency since Joseph Smith was also the first Mormon to actually stand a serious chance.