The Spaniard fell down the pecking order in the 2021-22 season, with Ben White and Gabriel Magalhaes forming Mikel Arteta's first-choice central partnership at the back.
Mari subsequently spent the second half of the campaign on loan at Udinese - scoring two goals in 15 appearances - before returning to North London for the pre-season period.
However, with William Saliba belatedly making his first-team breakthrough, Mari remained on the fringes of Arteta's setup, and he was loaned out to Monza for the 2022-23 campaign.
Establishing himself as a regular fixture in the Monza backline, Mari helped the Milan-based side retain their Serie A status, and his loan deal reportedly contained an obligation to buy if Monza avoided relegation to Serie B.
The 29-year-old has now taken to social media to bid farewell to the Arsenal faithful, expressing his "heartfelt gratitude" to "each and every one" of the Gunners' fanbase for their support.
"As my contract with @arsenal comes to an end, I wanted to take a moment to express my heartfelt gratitude to each and every one of you," Mari wrote on Instagram.
"Thank you for giving me the opportunity to represent this great and historic club, for all the memories we lived together and for the support I always received.
"It has been an immense honor to wear the Arsenal jersey and win two titles with this club. The Gunners will always have a place in my heart!"
Mari leaves Arsenal with a record of one goal from 22 matches in all competitions for the Gunners, netting his solitary strike against Rapid Vienna in the Europa League in December 2020, while winning one FA Cup and one Community Shield.
The sale of the defender will reportedly pocket Arsenal €4.9m (£4.2m), a slight loss on the £6.8m they paid Flamengo for his signature in the summer of 2020, six months after he initially joined on loan.
Last season, Mari contributed one goal from 31 games in a Monza shirt, despite memorably being stabbed alongside five other people - one of whom was killed - during an attack at a Milan supermarket in October 2022.
Mari survived with non-life threatening injuries and made a rapid recovery from surgery to return to action in January, helping Monza achieve an 11th-placed finish in the Italian top flight.
The Spaniard's move to Monza sees him become Arsenal's first senior sale of the summer window, following Ainsley Maitland-Niles out of the door after the Englishman's exit on a free transfer.
The likes of Rob Holding, Auston Trusty, Alex Runarsson, Granit Xhaka, Nicolas Pepe, Folarin Balogun, Thomas Partey, Albert Sambi Lokonga, Kieran Tierney and Cedric Soares could also be allowed to depart in the coming weeks as part of Arteta's latest clearout.