He began his military service in 1845 as a non-commissioned officer in the Life Guards Preobrazhensky Regiment. He was a participant in the Crimean War of 1853–1856 and directly involved in the defense of Sevastopol. He also took part in suppressing the Polish uprising. In 1864, he transferred from the army to the gendarmerie service, being appointed chief of staff of the Separate Corps of Gendarmes, then deputy chief of gendarmes, chief of gendarmes, and head of the Third Department of His Imperial Majesty's Own Chancellery.

Here is how the last day of General Mezentsev's life is described:

“Nikolai Vladimirovich usually got up very early and took daily walks on foot, during which he visited the chapel near Gostiny Dvor on Nevsky Prospect. Having stopped there on August 4th at nine o’clock in the morning, after finishing his prayers, Nikolai Vladimirovich, accompanied by his former comrade, retired Lieutenant Colonel Makarov, headed back home through Mikhailovskaya Street, Mikhailovskaya Square, and Bolshaya Italianskaya Street. As soon as he stepped onto the pavement of Italianskaya Street and reached the house of Kochkurov, right in front of the windows of a confectionery, two well-dressed men approaching him suddenly appeared. One of them inflicted a wound with a dagger and, together with his companion, hastily got into a carriage waiting nearby. Mr. Makarov tried to detain them, but a shot was fired at him from a revolver; the bullet missed, and the perpetrators, not stopped by anyone since there was not a single police officer in the area nor any bystanders, managed to escape. It is said that they quickly rode along Italianskaya Street and turned onto Malaya Sadovaya; the coachman was likely one of their accomplices. It is also said that they had appeared at the same place on previous days. It is certain that the criminals had their own carriage, harnessed to a fine horse with silver harness. Some conclude from these signs that they were men of means. One of them wore a gray coat. Nikolai Vladimirovich himself did not lose his presence of mind at the moment of the attack, and when the confectionery workers, who rushed out at the sound of the shot and were horrified, asked, ‘Who is wounded?’ Nikolai Vladimirovich answered that he was wounded, pointing to his bloodstained clothes. With the help of Colonel Makarov and chamberlain Bodisko, who came out of a neighboring house, Nikolai Vladimirovich Mezentsev walked along Italianskaya Street to the corner of Malaya Sadovaya, where he was put into a cab. From there, he reached his apartment near the Chain Bridge on the Fontanka River. Significant blood loss soon weakened the wounded man. Doctor Mamonov, called at 11 a.m., examined the patient and found his condition serious. Indeed, despite the assistance provided by doctors led by the then-famous surgeon Bogdanovsky, at 4 p.m. severe pains opened in the wound area and the stomach cavity, and at 5:15 p.m., in terrible agony, Mezentsev died.”

The attack on August 4, 1878, at Mikhailovskaya Square (now Arts Square) in Saint Petersburg was carried out by

Sergey Mikhailovich Stepnyak-Kravchinsky. The intention to kill the head of the political police of the Russian Empire, N. V. Mezentsev, arose in Kravchinsky after the trial of the 193 and the hunger strike of his friends in the Peter and Paul Fortress. According to the narodnik A. F. Mikhailov, Kravchinsky initially planned the assassination as an immediate duel on a square in Petersburg. His comrades convinced Kravchinsky to change this plan. For the assassination, an open carriage and a black trotter named Varvar were prepared—the same horse on which P. A. Kropotkin escaped from the Nikolaev Military Hospital. Kravchinsky chose a four-edged dagger as his weapon, which he brought from Italy. The narodniks A. I. Barannikov and A. F. Mikhailov also volunteered to participate in the assassination, with the latter assigned the role of coachman. According to Morozov’s recollections, Kravchinsky several times stepped out to meet the walking Mezentsev but did not dare to raise his hand against the chief of gendarmes. The immediate trigger for the crime was the execution of I. M. Kovalsky in Odessa. He was shot on August 2, 1878, for revolutionary activity and armed resistance during arrest. The next day, August 3, the news of the execution appeared in newspapers, and on August 4, 1878, on Italianskaya Street in Petersburg at nine o’clock in the morning, Sergey Kravchinsky carried out the attack.

An interesting fact: Sergey Mikhailovich Kravchinsky became the prototype for the hero of Lillian Voynich’s novel “The Gadfly.”

Sources:

Glinsky B. B. The Revolutionary Period of Russian History (1861–1881): Historical Essays. Part II. — St. Petersburg, [Printing House of A. S. Suvorin Partnership], 1913. pp. 251–252.

Pamphlet by S. M. Kravchinsky “Death for Death,” written by him after the assassination of Chief of Gendarmes Mezentsev.

https://dic.academic.ru/dic.nsf/ruwiki/133727

https://scepsis.net/library/id_2984.html