Born to a large family in northern Germany, Siemer was ordained as a priest of the Dominican Order in 1914 and studied philosophy, theology, philology and history.

He was appointed rector of the Order's high school in Vechta in 1921, and became Provincial of the Dominican Province of Teutonia (roughly, the whole of Germany north of Mainz) in 1932.

The Gestapo arrested Siemer in Cologne in 1935, as part of the "Currency Fraud Cases" targeting Catholic clergy and held him in custody for several months.

Siemer was influential in the Committee for Matters Relating to the Orders, which formed in response to Nazi attacks against Catholic monasteries and aimed to encourage the bishops to intercede on behalf of the Orders and more strongly oppose the Nazi state.

Following the failure of the 1944 July Plot to assassinate Hitler, Siemer evaded capture by the Gestapo at his Schwichteler monastery, and hid out until the end of the war, thus remaining one of the few conspirators to survive the purge.