In 1867, "he entered the military service of Alexander II as a lieutenant in the czar's infantry guard." "He rose to command the sentries at the czar's Winter Palace and the home of Crown Prince Alexander III," but "after four years of military duty, he was old enough to own his family estates [and so] "he resigned his commission as a captain and lived as a country squire." He married Raisa Borisenko, who, reportedly, was "also an orphan brought up by relatives." "He spent the 1870s selling off the trees of the estates' dense woodland and converting his land to agriculture." "Elected by gentry peers as county marshal of nobility, he became an outspoken writer and active in his rural government." "Never adopting Marxist or radical notions, Demens sympathized with populist leaders."
He became outspoken about the Czarist regime when "radical terrorist groups murdered Alexander II [and] his son tightened his political pressure on radicals and abandoned his father's reforms." He thus left Russia following the assassination of Czar Alexander II in 1881. Reportedly, many of the accounts of Demens being forced to flee Russia are "based on Demens' attempts to romanticize his departure from Russia by implying he escaped just before a military raid on his estate" despite never being "more than 'on the fringe' of peaceful populist organizations." Accoding to the biographer Albert Parry, Demens' departure was "more likely [due to] his troubles with an embezzlement scandal that engulfed Demens and others in government posts" ("trials brought only one conviction for a minor government official and acquittals for Demens and 15 others").
On June 8, 1888 the first train pulled into the terminus in southern Pinellas county (the end of the line) with one passenger. The area had no official name and no real streets or sidewalks. Demens named the location of his terminus St. Petersburg, Florida, after Saint Petersburg, Russia, where he had spent half his youth.
Demens eventually retired to Alta Loma, California to the family ranch (what later became known as Demens-Tolstoy Estate). Reportedly, "the descendants of Peter Demens now live in California and British Columbia, [including] a grandson, Peter Demens Tolstoy ("writer Leo Tolstoy, who wrote Anna Karenina and The Death of Ivan Ilych, is his great-grand uncle), a great-grandson, Greg Demens, and a great-great-grandson, Greg Demens." Demens Landing in St. Petersburg, FL is named in his honor.