He rose to the rank of major-general, but became famous by being the type par excellence of the corrupt and egoistic Swedish parliamentarian of the final period of the Age of Liberty; he received for many years the sobriquet of "General of the Riksdag".

[1] Pechlin first appears prominently in Swedish politics in 1760, when by suddenly changing sides he contrived to save the Hats from impeachment.

In 1786, when the opposition against Gustavus III was gathering strength, Pechlin reappeared in the Riksdag as one of the leaders of the malcontents, and is said to have been at the same time in the pay of the Russian court.

In 1789 he was one of the deputies whom Gustavus III kept under lock and key until he had changed the government into a semi-absolute monarchy.

On the eve of the assassination (16 March) the principal conspirators met at his house to make their final preparations and discuss the form of government which should be adopted after the king's death.