Born in Grevenmacher on the Moselle in 1858, Seimetz initially worked as a house painter and decorator until the industrialist Antoine Pescatore discovered his artistic talents and became his patron. In 1875, following an apprenticeship, Seimetz moved to Brussels to study painting, continuing his studies at the Munich Academy of Fine Arts (1879–1881) and finally at the Ecole des Beaux-Arts in Paris (1881–1887).
From 1887 to 1896 Seimetz traveled to the United States, Canada, the West Indies and Mexico. In the United States, he sailed down the Mississippi on a flat boat. In 1904, he was awarded the Grand-Duke Adolphe Prize, together with painter Dominique Lang. Seimetz was the first Luxembourger to delve into Impressionism and the first, after Nicolas Liez, to paint out in the open air.