Mawhinney will join as a senior art director. Donnelly has been working alone since his partner, the art director Ken Grimshaw, died in an accident last year.
A date has yet to be decided for Mawhinney's arrival at JWT, and there are no immediate plans to replace him at Faulds. The joint deputy creative directors, Steve Mawhinney and Pete Bastiman, will run the department in the meantime.
Faulds' creative department has also been boosted by the recent addition of Tony Malcolm and Guy Moore following the agency's acquisition of Malcolm Moore Deakin Hutson.
Billy Mawhinney started his career at JWT as a trainee art director in 1978. After two years he moved back to Belfast, but returned to the agency in 1982 where he first started working with Nick Welch on accounts including Persil, Kit Kat and Kellogg.
They left in 1985 to join Bartle Bogle Hegarty, but returned to JWT four years later.
The duo left JWT as creative directors in 1995 and, after a nine-month flirtation with CDP, moved to Still Price Lintas, where they separated.
Mawhinney then spent a year at Ogilvy & Mather and Euro RSCG Wnek Gosper respectively before moving to Scotland to join Faulds in 1999.
Jaspar Shelbourne, the executive creative director of JWT, said: "Billy has always retained strong links with the agency. On a personal level, he's an extremely charming person to do business with, and he and John are extremely good strategic presenters and very good with clients."
Mawhinney added: "After the initial gulp, I can't wipe the smile off my face. He added: "When I moved to Faulds, I gambled with my family, but I'm proud of my three years here."