From the start of his career Leys painted history and genre subjects. During this period Leys often collaborated with the Belgian Romantic painter Gustaf Wappers (1803-1874). Both artists were interested in nationalistic subjects painted in styles that owe much to the example of 16th- and 17th-century Flemish painting. In 1835 Leys went to Paris where he visited the studio of Eugène Delacroix and met Paul Delaroche. The influence of Delaroche's Romanticism is evident in Leys' early work. His precocious talent was manifested at the Brussels Salon of 1836 where he exhibited his "Massacre of the magistrates of Louvain" for which he received high praise.