Back in Finland he took up a position at a government agency overseeing the construction of public buildings in 1877. He made a successful career at the agency and received a new and higher appointment there as late as 1910. Early in his career he became involved in the preparations of Finland's contributions to the world fair in Paris in 1878 and another exposition in Copenhagen in 1888, together with Robert Runeberg and Julius af Lindfors. He thus got an opportunity to develop his talent not only as an architect but also as an artist and designer. He was a close friend of artist Fanny Churberg and associated with the Friends of Finnish Handicraft group (engaged in designing and making textiles) although he later largely parted ways with this group. In 1885 he was responsible for the planning of the visit to Lappeenranta by Emperor Alexander III for a military exercise and also supplied the designs for the interior of a fishing cabin that was given as a gift to the Emperor. Ahrensberg's background in multicultural Vyborg and skill in languages may have made him appear suitable for jobs related to the Russian authorities (Finland was at the time an autonomous part of the Russian Empire).