He amassed a considerable fortune (4.4 million rubles), consisting of real estate (5 houses in Moscow), securities, money and bills.
Tretyakov started to collect art in 1854 at the age of 22; his first purchase was 10 canvases by Old Dutch masters.
In his collection Tretyakov included the most valuable and remarkable products, first of all the contemporaries, from 1870 - mainly members of the society of circulating art exhibitions (Tovarishchestvo peredvizhnyh hudozhestvennyh vystavok or Peredvizhniki, Передвижники in Russian).
He also acquired Viktor Vasnetsov’s collection of the sketches made during his work on St Volodymyr's Cathedral in Kiev.
Aspiring to show the beginnings and development of the domestic school of art, Tretyakov began to acquire pictures by masters of the 18th and first half of the 19th centuries and landmarks of Old Russian painting.
The collection in Tretyakov's gallery was equal in importance with the largest museums in Russia at that time, and became one of sights of Moscow.
Tretyakov continued to build up his collection, for example, in 1894 he donated a gallery of 30 pictures, 12 figures and a marble statue “The Christian martyrs“ works by Mark Antokolski.
He supported an initiative to help the families of soldiers who died during the Crimean and Russo-Turkish War.
He never refused monetary help to artists and others, and carefully looked after the affairs of painters who without fear entrusted their savings to him.
With his brother Sergey he supported the Moscow School for the Deaf, founded in 1860 by the artist Ivan Karlovich Arnold, and became its patron.
Pavel Tretyakov died in December 1898, and his funeral was held at the Church of St. Nicholas in Tolmachi where he was an active parishioner.
He provided for the financing of the gallery, and also for a shelter for the widows, juvenile children and unmarried daughters of artists who had died.