After finishing his studies, Karl was invited on a trip to Surinam by the explorer August Kappler. In November 1845 the two departed from the port on the island of Texel aboard the ship "Natalie." After a dangerous voyage, they reached New Guinea in mid-March 1846. From Paramaribo they went into the interior of the country, where Mohr collected plants for European florists and herbariums. Severe fever attacks struck Mohr and tied him to a sick bed for several months, so he felt compelled to follow the advice of the doctors and returned home in November 1846.