Mount St. Helens last erupted in a dome-building eruption that started in October 2004 and lasted until early January 2008. This eruption began with a swarm of earthquakes on September 23, 2004, and the volcano emitted plumes of ash and steam starting October 1, 2004, with continued dome growth and small earthquakes throughout that period.

The most famous previous major eruption occurred on May 18, 1980, when a massive explosive eruption took place following a two-month series of smaller explosions and earthquakes. This 1980 eruption was highly destructive but was followed by a long period of dormancy until the 2004-2008 activity.

So, the last eruption of Mount St. Helens was from 2004 to early 2008.