Pyotr Dementyev was born into a noble family and spent his childhood in an ancient estate in the Tver province. He studied in a school in St. Petersburg, and then served for three years as an officer in the Yeger Guard Regiment. Rising to the rank of staff captain and serving as the battalion commander, Dementyev unexpectedly resigned and returned to his estate near Vseyegonsk, where he engaged in agriculture. Soon he became a zemstvo leader, and in 1879 he was elected the head of the nobility of the Vseyegonsk district.

In the 1870s, in search of more effective farming methods, he visited America several times. In 1881, after the assassination of Alexander II and accusations of connections with the Narodnaya Volya, Dementyev decided to emigrate to the USA. He left with a small amount of money and settled in Florida, where he engaged in timber harvesting. He recalled the first year of life in this state, where there were only 800 white settlers, as a period when he had to fight for survival and provide for his family.

Later, he took on railway contracts and decided to build a 150-mile railroad from the St. Johns River to the Gulf of Mexico, crossing Florida from east to west. New settlements appeared along the under-construction road, one of which was named Odessa in honor of Dementyev.

On June 8, 1888, the first train reached the shores of the Pinellas Peninsula in Tampa Bay. Here, Dementyev built a railway station, a hotel, and a pier, naming the new city St. Petersburg in honor of the Russian capital.

The exhausting work and hot climate of Florida took a toll on Dementyev's health, and in 1889 he left St. Petersburg. After three years in Asheville, North Carolina, the family moved to Los Angeles. "I have gained and lost significant sums more than once, but I was always supported by Russian endurance and resourcefulness," Pyotr Alekseyevich admitted. In 1893, he became a naturalized American citizen under the surname Demens.

Dementyev visited Russia twice, in 1896 and 1906, and met with P. A. Stolypin. He also corresponded with the Ober-Procurator of the Synod K. P. Pobedonostsev on issues related to the resettlement of Doukhobors.

In his later years, he lived on his Alta Loma ranch near Los Angeles. At the beginning of the world war, he participated in the work of the Russian government procurement commission in Washington. Pyotr Alekseyevich Dementyev passed away in 1919.

In 1979, a granite pedestal was erected in St. Petersburg in memory of its founder, who named the city after Petersburg.