Samuel Langhorne Clemens was a 19th century author, humorist, and occasional moralist. He was most noted for his literary works written under the pseudonym 'Mark Twain'.
In the mid 1860s Samuel Langhorne Clemens was visiting San Francisco in 1893 at the age of 58. Clemens knew Guinan during this time and they ran in some of the same circles. By that time he develeoped disenchanted views and while attending the same party with Guinan, he derided the romantic views that claimed Earth was created for the sole purpose of hosting humankind.
A time-stranded Data crashed the event, believing Guinan had come back in time to find him. Clemens overheard part of their conversation and became convinced that Data had traveled to the past for nefarious reasons. (TNG: "Time's Arrow")
He tried to relate his theories to a young reporter. He stalked Guinan and followed Data's movements, aided by Ches Whitley. He even paid Jack London the bellboy to open Data's room in Hotel Brian where he witnessed his temporal transceiver; during their conversation he urged the bellboy to pursue his dreams and write about them. Not long later, Data and Guinan found him in the closet, and threatened that he would warn humanity about the impending invasion.
After hearing about a confrontation of two people "vanishing" and some confrontation in a hospital, he traveled to Presidio and invited the reporter for the story of his life. Armed with a Colt .45 he confronted Captain Picard and his command crew, determined to surrender them to the authorities. But then the Devidian couple arrived from a portal, Data was decapitated and Clemens followed everyone into the portal, finding himself on Devidia II of 2369.
He eventually ended up being transported to the USS Enterprise-D and took a tour. He was convinced by counselor Deanna Troi that the future had many positive merits. When it was learned that the Devidian temporal rift would only allow one person to go through it in either direction one more time, Samuel Clemens was returned to his time to rescue Captain Picard trapped in the past. After a briefing, he returned to Market Street and reached the cave where Picard and Guinan remained trapped; he supplied Picard with the Devidian cane and instructions on how to activate the ophidian and return to the future.
As Picard and Clemens parted ways, Picard wished that time had allowed him to know Clemens better, while Clemens assured Picard that he could always read his books. With Picard having departed, Clemens took care of the injured Guinan until help arrived, as well as leave his watch with Data's head to wait to be found in 500 years later; and for history to repeat itself. (TNG: "Time's Arrow, Part II")
Clemens was also credited with responding "Reports of my death have been greatly exaggerated" upon reading his prematurely printed obituary. This quote has been paraphrased by Picard after his life-saving heart surgery in 2365, as "Any rumors of my brush with death are greatly exaggerated" (TNG: "Samaritan Snare"), also by Picard during a Borg attack in 2373 as "Reports of my assimilation have been greatly exaggerated" (Star Trek: First Contact); by Kasidy Yates referring to her morning sickness in 2375 as "Reports of my death have been exaggerated... but not by much" (DS9: "What You Leave Behind"); and by The Doctor in 2377 as "The reports of my decompilation have been greatly exaggerated." (VOY: "Body and Soul")
Clemens was played by actor Jerry Hardin. His makeup for the role involved work by Doug Drexler. (Star Trek: Communicator issue 136, p. 31) With the encouragement of director Les Landau, Hardin recreated the role for a successful one-man stage show not long after the TNG episodes featuring him.
It is interesting to note that in 1893, Samuel Clemens was traveling on book tours, mostly in Europe. He only returned to the US briefly (New York City) and moved his family to Berlin, Germany. It is unlikely that he was in the San Francisco area at all.
Coincidentally, both his birth and death coincided with the passing of Halley's Comet, which he asked about aboard the Enterprise.
The books which Clemens completed, or wrote, after his 1893 encounter with the Enterprise included Pudd'nhead Wilson, Personal Recollections of Joan of Arc, The Mysterious Stranger, and his autobiography.