After graduating from Hosei University, Iwago went as his father's assistant to the Galápagos Islands, where he decided to become a photographer.
His work Letters from the Sea (海からの手紙, Umi kara no tegami) won the Kimura Ihei Award in 1979.
He is also the first Japanese photographer whose work has twice appeared on the cover of National Geographic (May 1986 and December 1994).
Since 2012, Iwago has appeared on the TV program "MITSUAKI IWAGO'S WORLD "CATS" TRAVELOGUE [jp](IWASGO MITSUAKI NO SEKAI NEKOARUKI, (岩合光昭の世界ネコ歩き)" (NHK BS Premium).
In the TV program, Iwago is shooting videos of cats from all over the world.