He was chairman of the Swedish Municipal Workers' Union 1977–1988 and governor of Örebro County 1989–1994.

He earned a reputation for his quick replies, and was sought after as an interviewee in the press, where his statements were often given negative attention.

Upon his retirement in 1994, he was appointed chairman of the newly created Granskningskommissionen, a council reviewing the police investigation of the assassination of prime minister Olof Palme.

In 1996, Marjasin was suspected of having submitted manipulated receipts and double invoicing expenses during his time as governor.

The media attention was most intense during the summer of 1996, when Marjasin, with regard to the ongoing police investigation, did not want or could not speak to the press.