Demens was born in May 1850 as Pyotr Alexeyevitch Dementyev to a wealthy family in Vesyegonsky District, Tver Oblast, in the Russian Empire.

Demens was a liberally minded, well-educated aristocrat, a first cousin of Prince Petroff and a captain in the Russian Imperial Guard.

Demens was raised by his maternal uncle Anastassy Alexandrovich Kaliteevsky, marshal of the Vesyegonsk district nobility, who became the boy's tutor and guardian of his land estates.

[2] He rose through the ranks to command sentries at the czar's Winter Palace and the home of Crown Prince Alexander III.

[2] Some sources state that Demens became outspoken about the czarist regime and left Russia following the assassination of Czar Alexander II in 1881.

[citation needed] He decided to enter the lumber business ("investing in a sawmill and a construction company in Longwood, Fla."[5]), and in 1885 Demens was supplying railroad ties to the narrow-gauge Orange Belt Railway.

As owner of the railroad, Demens "[extended] its lines to link Kissimmee with Jacksonville and Tampa Bay"[2] (with the help of Hamilton Disston).

Demens co-founded St. Petersburg, Florida, with John Constantine Williams Sr. On June 8, 1888, the first train pulled into the terminus in southern Pinellas County (the end of the line) with one passenger.