In 1889, Rex participated in an exhibition of the Munich Artists' Association. He returned to Prague around 1890. In 1891, a drawing of him at the Annual Exhibition of Visual Artists of Vienna was displayed. One of Rex's works was awarded a bronze medal at the 1900 World Exhibition in Paris. Shortly before the First World War, an exhibit of about 30 of his paintings depicting scenes from the life of Napoleon Bonaparte was displayed in 38 cities.