The Senegal international has experienced a marked downturn in fortunes over the past 12 months and lost his number one status to Kepa Arrizabalaga last season.
A serious finger injury restricted Mendy to just 12 appearances in all competitions, including nine starts in the Premier League, during which he kept just one clean sheet on the opening day of the season against Everton.
Mendy's only top-flight appearance in 2023 so far saw him ship two goals in a 2-2 home draw with Nottingham Forest on May 13, and he was criticised for his part in Taiwo Awoniyi's opener on the day.
The former Rennes man still has two years left to run on his Blues contract, but he has apparently rejected the club's offer of an extension and will be allowed to seek pastures new this summer.
Tottenham Hotspur are thought to have identified Mendy as a potential successor to Hugo Lloris, but according to Fabrizio Romano, a move to Saudi Arabia is on the verge of completion.
Romano reports that the 31-year-old has now given the green light to a three-year contract with Al-Ahli, who are now in the final stages of negotiations with Chelsea over a fee.
Mendy's switch to the Saudi Pro League - which will be the latest in a long line of high-profile moves to the Gulf State - will bring an end to a successful three-year spell at Chelsea since joining for £20.5m in the summer of 2020.
The Senegalese shot-stopper ousted Kepa and won the Champions League in his debut season at Stamford Bridge, keeping an all-time record nine clean sheets throughout the tournament.
Mendy's European exploits won him Senegal's Player of the Year award for 2021, and he also has winners' medals in the UEFA Super Cup and Club World Cup under his belt from his time in the English capital.
From his 105 appearances in a Chelsea jersey, Mendy has kept 49 clean sheets, and his departure will likely mean that Kepa stays at the club as a backup to a new number one.
Inter Milan's Andre Onana was previously touted as a top target for Mauricio Pochettino, who was also believed to have taken a liking to AC Milan and France goalkeeper Mike Maignan.
However, the Blues are said to have cooled their interest in both players due to the Milan clubs' asking prices, while Brentford's David Raya and Valencia's Giorgi Mamardashvili are alternative options.
Leeds United's Illan Meslier has also been linked with a move, but speculation has died down since his torrid end to the season with the Whites, who were relegated back down to the Championship for the 2023-24 season.
Meslier has also affirmed that he would not be prepared to be a number two at Chelsea, who could also offload Kalidou Koulibaly and Hakim Ziyech to Saudi clubs in the coming weeks.