The Spanish champions underwent tests on Monday ahead of the team's return to training on Tuesday after a week off. Mariano, 26, is asymptomatic and isolating at home.
The tests done on Monday were carried out at the players' houses and hence, Mariano was not in contact with any other players or staff. Mariano is set to miss Madrid's Champions League round-of-16 return leg at Manchester City on Aug. 7. Madrid take on City at the Etihad stadium needing to overturn a 2-1 first-leg deficit.
A club statement read: "After the COVID-19 tests carried out yesterday on our first team footballers by the Real Madrid Medical Services, our player Mariano, tested positive. The player is in perfect health and is complying with the protocol of isolation at home."
Mariano appeared on Instagram live on Tuesday, where he told viewers: "Thank you very much for your messages of support. I'm perfectly well. Thank God both family and my environment are great. I am confined at home and I hope to return to normal soon and return to my teammates and team."
Sources have told ESPN that Mariano spent the last week on holiday in Marbella, Spain, and was in contact with people outside of his entourage.
Most of the Madrid players opted to leave the Spanish capital and spend the holidays on the Spanish coast.
While Madrid are exempt from the 14-day period of quarantine upon entering the UK, they will need to remain in isolation for the duration of the time that they are in England, except to participate in training and in the match itself.
ESPN has approached UEFA for comment regarding the status of Real Madrid's match with City in light of Mariano's positive test.