In 1793, he graduated from the St. Petersburg Mining School and, as a shaft master, joined the Kolyvano-Voskresensky Altai factories. There, he engaged in surveying work and managed forests at various mines, including the Zmeinogorsky and Goltsovsky. In 1797, he oversaw supplies at the Suzun copper smelter, and from 1798 to 1801, he was responsible for the delivery of lead from the Nerchinsk to the Kolyvano-Voskresensky factories.

From 1801, he worked as a surveyor, then as a forest master at the Barnaul drawing office. In 1804 and 1805, he created maps of the Irtysh River and its tributaries. In 1806 and 1807, he proposed projects for horse-drawn railways and waterways, but they were not accepted. However, in 1806, his project for a cast-iron road from the Zmeinogorsky mine to the Korbalikha factory was approved and put into operation in 1809. In 1808, he supervised the construction of vessels for transporting ore to Barnaul and organized the state library.

From 1811, he headed the drawing office of the Department of Mining and Salt Affairs in St. Petersburg. In 1812, he explored the mouth of the Kama River for the construction of salt warehouses and studied options for transporting salt from Lake Elton to the Volga. In 1814, he inspected the Starorus salt factory, and in 1815, he investigated the reasons for peasant discontent at the Beloretsky factories.

In 1817, he became the head of the Kolyvano-Voskresensky factories district and in 1822, the Tomsk civil governor. With his assistance, new productions were introduced at the Guryev factory, coin minting and metal smelting were increased, and a paper factory and printing house were built. In Barnaul, the construction of several public buildings began, including a hospital and a museum. In 1823, the Local History Museum was founded, for which models of machines and mechanisms were made.

In 1830, Pyotr Kozymich retired and moved to St. Petersburg. In 1831, he returned to service as a secret advisor and senator. In 1837, he became a member of the temporary council for the management of state property and chairman of the commission for confiscated estates. Since 1838, he headed the commission for managing state properties in the western provinces.

He was a well-known collector of ancient Russian books and works of art, some of which he donated to museums and libraries. He passed away in 1839 and was buried at the Tikhvin Cemetery of the Alexander Nevsky Lavra.