Born in Saint Petersburg or Moscow, he was the second surviving son of Count Nikolai Nikitich Demidov and Baroness Elisabeta Alexandrovna Stroganova.

In 1837–38, he organised a scientific expedition of 22 scholars, writers and artists, of which Auguste Raffet and the critic Jules Janin became Demidov's friends, to southern Russia and the Crimea, headed up by Frédéric Le Play.

[2] Demidov also financed a trip to Russia by André Durand in order to identify landscapes, which were published under the title Voyage pittoresque et archéologique en Russie in 1839.

In 1847 Demidov made a trip to Spain with Raffet, publishing an account of it later as Etapes maritimes sur les côtes d’Espagne in 1858.

In the Paris Salon of 1834 he acquired Paul Delaroche's The Execution of Lady Jane Grey, at the present in the National Gallery, London.

In 1839, he was introduced by Jules Janin into the circle of Jérôme Bonaparte, former king of Westphalia, who was living in exile at the Villa di Quarto in Florence.

In March 1841, the couple went to Saint Petersburg, where the Tsar was full of attention for his first cousin Mathilde, through her mother, and losing no opportunity to humiliate Demidov by any means possible.

On 17 August 1841, the couple arrived in Paris, where they lived at hôtel Demidoff at 109 rue Saint-Dominique until June 1842, when they moved to spend a year in Saint Petersburg before finally setting up home at the Villa San Donato.

In many ways, Demidov felt he deserved such punishment, and their separation was authorised in 1847 by a personal decision of tsar Nicholas I. Demidov's many other mistresses included Maria Calergis, considered one of the most beautiful women of her era, Ernestine Duverger, and Fanny de la Rochefoucauld (daughter of François, 8th Duke of La Rochefoucauld, with whom he had an illegitimate son.