During a 20-year career, he became synonymous with spectacular underwater ads for Speedo, John West and the British Gas "babies" film that won him a number of awards.
He will probably be best remembered for one of his final commercials - the 1996 spot for Cheltenham & Gloucester featuring a small boy diving for a pearl in a sunken temple.
Portelly began his working life as a dentist, but soon developed a passion for diving and underwater photography, which he exploited at Portelly Films, the company he established with the producer Amos Manasseh.
"To an outsider, many of the films we made were impossible to conceive," Manasseh said. "But on the inside, Mike exuded a sort of calm confidence which would inspire the whole team."
Over the past ten years, Portelly became a strong advocate of cause-related advertising as a means of funding conservation projects.
Portelly's funeral will take place on 20 July at St Peter's Church in Swindon.