Johnson is hosting a three-day summit of the Group of Seven (G7) leaders at the English seaside resort of Carbis Bay. The representatives of the European Union are also in attendance.
When Johnson, Von der Leyen and EU Council president Charles Michel took their seats on Saturday, Johnson said "I'm delighted that all the protocols seem to have gone alright so far, and we've got the chairs".
Von der Leyen responded, "impressive, who would've thought of that".
The joke referred to a meeting in April between Von der Leyen, Michel and the Turkish President Tayyip Erdogan in Ankara when the EU Commission president was given a seat on an adjacent sofa.