City began pre-season training at the end of August and have been slowly welcoming some of their stars back from international duty, with all players regularly taking tests to ensure they remain safe to train.
Neither Mahrez nor Laporte were part of that international cohort, but City confirmed on their official website that the pair have tested positive and will now enter a period of self-isolation until they can successfully pass a test. Both players are said to be in good health and displaying no symptoms.
City have been given an extended pre-season as a result of their deep run in the Champions League this summer, meaning their first fixture of the new season - the visit of Aston Villa - has already been postponed.
Instead, City's season will get underway on September 19 with a trip to Molineux to face Wolverhampton Wanderers, but whether either Laporte or Mahrez will be available for that game remains to be seen.
On top of the obvious health concerns, this news will come as a huge blow to City from a footballing perspective, with Laporte's unavailability likely to be a major issue for manager Pep Guardiola.
When fit, the Frenchman was City's only undisputed starter in defence last season. A knee injury robbed him of the majority of the campaign, but he still recovered in time to finish the season with 15 appearances in the Premier League.
Guardiola was always eager to call on Laporte, having lost faith in both Nicolás Otamendi and John Stones. Defensive midfielder Fernandinho was often converted into a defender, while young Eric García was brought through to help manage the workload.
City have brought in Nathan Aké from Bournemouth and are still on the hunt for a new centre back, with Napoli's Kalidou Koulibaly understood to be at the top of their wish list.