36°27'57.0"N 114°22'13.0"W

St. Thomas is one of the earliest settlements in the state of Nevada.

The first real settlement in the Southern part of the County was in 1856 when a number of Mormon families settled at Las Vegas - The Meadows. In 1857 when the San Bernardino Branch of Mormons was called to Salt Lake, the station fell into the hands of Gentiles who have occupied it since. In 1865 a mission of about 250 Mormon families from Utah settled the Muddy Valley, establishing the towns of St. Thomas (originally laid out under instructions from Brigham Young in 1864), Overton, St. Joseph, Junctionville and Bunkerville. At the period of its greatest prosperity, in 1867, St. Thomas contained about 500 inhabitants. These Mormons considered themselves residents of Pahute Co., Arizona, but the subsequent session by Congress of a degree of longitude from Arizona to Nevada put them indisputably in Nevada. Controversies then arose about back-taxes, to end which Brigham Young ordered the abandonment of the settlements in the Muddy Valley. March, 1871, witnessed the exodus of the entire population excepting one family.
-1913 Lincoln County History,by A. L. Scott.
In fact, it was a happening place when John Wesley Powell made his famous excursion down the Colorado River in 1869.

We camp at noon under the right bank. And now, as we push out, we are in great expectancy, for we hope every minute to discover the mouth of the Rio Virgen. Soon one of the men exclaims : "Yonder's
an Indian in the river." Looking for a few minutes, we certainly do see two or three persons. The men bend to their oars, and pull toward them. Approaching, we see that there are three white men and an Indian hauling a seine, and then we discover that it is just at the mouth of the long sought river. As we come near, the men seem far less surprised to see us than we do to see them. They evidently know who we are, and, on talking with them, they tell us that we have been reported lost long ago, and that some weeks before, a messenger had been sent from Salt Lake City, with instructions for them to watch for any fragments or relics of our party that might drift down the stream. Our new found friends, Mr. Asa and his two sons, tell us that they are pioneers of atown that is to be built on the bank. Eighteen or twenty miles up the valley of the Rio Virgen there are two Mormontowns, St. Joseph and St. Thomas. Tonight we dispatch an Indian to the last mentioned place, to bring any letters that maybe there for us. Our arrival here is very opportune.When we look over our store of supplies, wefind about ten pounds of flour, fifteen poundsof dried apples, but seventy or eighty poundsof coffee. August 31.—This afternoon the Indianreturns with a letter, informing us thatBishop Leithhead, of St. Thomas, and twoor three other Mormons are coming downwith a wagon, bringing us supplies. Theyarrive about sundown. Mr. Asa treats uswith great kindness, to the extent of his ability; but Bishop Leithhead brings in his wagon two or three dozen melons, and many other little luxuries, and we are comfortable once more. September 1.—This morning Sumner, Bradley, Hawkins, and Hall, taking on a small supply of rations, start down the Colorado with the boats. It is their intention to go to Fort Mojave, and perhaps from there overland to Los Angeles. Captain Powell and myself return with Bishop Leithhead to St. Thomas. From St. Thomas we go to Salt Lake City.
-FIRST THROUGH THE GRAND CANYON By major JOHN WESLEY POWELL Being the Record of the Pioneer Exploration of the Colorado River in 1869-70, Published 1915

Besides Mormons farming, there was also some mining activity in the area.

The St. Thomas or Virgin River district comprises a large, vaguely defined area in the vicinity of St. Thomas, Nev. The town of St. Thomas has been abandoned, as eventually it will be flooded by water impounded by Bounlder Dam. The district is particularly rich in industrial mineral deposits, including silica sand, magnasite, gypsum, salt, and sodium sulphate; but due to their inaccesibility and distance from consuming centers, only silica sand and salt have been produced. THe salt deposits have beenworked mainly to supply local demand. THe Los Angeles & Salt Lake R.R., a part of the Union Pacific system, passes through Moapa, Nev., 14 miles northwest of Overton. From Moapa a branch railroad runs southeast to St. Thomas, a distance of 21 miles. ABout five miles of the southern part of this branch line will be abandoned when the water from Lake Mead reaches its maximum flow line.
-RECONNAISSANCE OF MINING DISTRICTS IN CLARK COUNTY, NEV., Venderburg, 1937

Unfortunately for those who found its setting idyllic, progress was to end the town's existence.

ITS LAST CENSUS
ST. THOMAS, NEVADA, down in the Moapa valley, is making its last appearance in any census. Its population of 234, an increase of sixty-four from ten years ago, will have to scatter to other localities when the Boulder dam is completed, because the dam will back up the Colorado river, and St. , Thomas will lie under from ten to fifty feet of water. It is one of the old settlements of the state, as it was established by the Mormons even before they founded a church stake at Las Vegas. Genoa in the Carson valley was founded in June, 1850, by Mormons from Salt Lake City, and Mormons settled at Franktown in Washoe valley as early as 1847, according to Thompson & West's history. What is now St. Thomas was the home of Mormon farmers in the early 'fifties, but these all left the place when Brigham Young recalled his followers to Utah in 1857. In 1864 they returned and St. Thomas has been a busy farming settlement ever since. To be swallowed up in the interest of progress is a fate that frequently overtakes villages. Only recently half a dozen Welsh , communities have been wiped out by a vast reservoir to supply the needs of London for potable water, and it is only a few years since Sleepy Hollow in New York, famous through Washington Irving's stories, ceased to exist. It is now the floor of an artificial lake that forms part of the water supply of New York City.
-Reno Eveing Gazette, 26 Apr 1930

And soon, the dreaded day arrived.

LAKE AT BOULDER PROJECT TAKING FORM
The world's largest man-made lake is taking form. Behind gigantic Doulder Dam's mass of concrete, Boulder lake is spreading through deep canyons and across deserts. Ultimately it will have shorelines of 550 miles and an area of 145,000 acres, making it a dozen times larger than the lake above Aswan dam on the Nile. WHile new vistas will be reveled, some familiar scenes will be submerged and added to legends of sunken cities or lost continents. The little vacated town of St. Thomas will meet a watery grave.
-Reno Gazette Journal, March 18. 1936

A note about the post office. Although St. Thomas has always been in Nevada, confusion about borders led folks to believe that it was in Utah, then Arizona. Clark County was formed taking part of Lincoln county in 1909.

Normally under the waters of Lake Mead, present (2022) drought conditions have exposed St. Thomas again. Having never visited, we can't tell you what kind of conditions you'll find, but it's been exposed long enough that there is probably no mud left. Since it's about 500 miles away, I doubt we'll be making the trip any time soon.