While in Bern, he became active as a mountaineer and skier. Mertz competed in several national competitions; in 1906 he was third in the Swiss cross-country skiing championship, and second in the German championship. In 1908, he won the Swiss ski jumping championship, with a distance of . As a mountaineer, he was particularly prolific in the Alps; he climbed Mont Blanc—the highest peak in the range—and claimed several first ascents of other mountains.