The newspaper said staff had bought a large drinks fridge for the office, which they refilled by taking a suitcase to the local supermarket to buy bottles.
The Mirror said Johnson attended a "handful" of the gatherings when indoor socialising was banned. A senior civil servant is investigating a spate of parties at Downing Street, and Johnson apologised for the events earlier this week.
(Reporting by Kate Holton; Editing by Leslie Adler)
