His first business in California was a steam-powered laundry. Dementyev quickly mastered the new equipment and, according to contemporaries, did not shy away from any work, although due to his aristocratic pride, he never stood behind the counter. Later, he became a co-owner of an agricultural products company, acquired shares in two local banks and the state railroad, and also became a co-owner of a shaving powder manufacturing enterprise. Soon he returned to sawmills and became president of the union of all sawmills from Canada to Mexico. The entrepreneur's daughter, Countess Vera Tolstaya, recalled that when her father retired, the sawmill owners gifted him a black wood cane with a gold handle and a commemorative inscription, as well as the chairman's hammers with inscriptions on silver plates.

During World War I, Dementyev sought to help Russia. In particular, in 1916, he offered his services to the Russian government purchasing commission in New York. His articles on the military successes and failures of the Russian army and the Entente army were regularly published in the Los Angeles Times.

Peter Alexeyevich Dementyev passed away on January 21, 1919, at his estate in Alta Loma. The next day, an obituary was published in the Los Angeles Times, calling him a well-known Russian patriot and writer, as well as a prominent financier and railroad builder in the USA. He was an expert in Russian and American affairs, interacted with many people from the financial world of the USA, and actively participated in the cultural life of his homeland. His talents and outstanding abilities attracted many people to him.