Before he became Governor-General Seyn had been a staff officer in the military district of Finland and an aide to the Governor-General. Seyn contributed in the Russification of Finland as he followed in the foot-steps of the former Governor-General Nikolai Ivanovich Bobrikov. Finnish autonomy was further limited and in laws passed in 1908 and 1910 the Duma, instead of the Finnish Diet, was given rights to make laws concerning Finland.
After the revolution the Provisional Government arrested Seyn on March 16 1917 and brought him to Saint Petersburg where he apparently was killed the next year.