Mullan, 31, joined Wasps in 2013 and made 128 appearances for the club as well as earning 17 caps for England, but was unable to feature for Wasps this season whilst recovering from a knee injury.
Wasps director of rugby Dai Young said: “The club wish Matt well with his next steps and thank him for his contribution to Wasps Rugby over the past five seasons.”
Meanwhile, winger Ambrose Curtis has left the club to join French Pro D2 side Vannes.
Curtis, 26, featured for Wasps A in the Premiership Rugby Shield and made one first team appearance in the Premiership Rugby Cup against Northampton Saints in November.
Young added: “We’d like to thank Ambrose for his efforts and professionalism during his time at Wasps and wish him all the best in the future.”