Alastair Wood, a group head at J. Walter Thompson, is moving to the agency and will be joined by Guy Moore and Tony Malcolm, two senior creatives from Wieden & Kennedy.
The appointments follow the departure of Thornton's creative partner, Paul Shearer, earlier this month.
Dan Fisher, a copywriter from WCRS, and Rick Brim, an art director at Rainey Kelly Campbell Roalfe/Y&R, are also due to start at the agency later this month.
Thornton said: "Guy and Tony are that very rare breed, incredibly talented, grown-up creative directors who've been there, done that and still want to go there and do it all over again.
"Alastair brings a wealth of experience both as a great writer and a great manager, and Rick and Dan have a fantastic amount of talent and experience for a team of such tender years.
"We were really sad to lose Paul, but these arrivals should ensure that not only does the work not suffer, but that it starts getting better and better."
Wood has been at JWT for the past four years and was named acting creative director in 2002 until Nick Bell was appointed as the executive creative director in May last year.
He has also worked at Lowe and at a host of other agencies in the UK and the US.
Moore and Malcolm have worked together for the past 19 years and in June 2002 formed a start-up, Malcolm Moore Deakin Blazye. This was acquired by Faulds in 2000, where the duo worked as the creative directors.
During a stint at TBWA they created the "parklife" commercial for Nike which picked up three D&AD silver Pencils.
Moore said: "Leo Burnett has a client list to die for and with Jim at the helm it was an opportunity too good to miss."
Fisher and Brim have worked across a wide range of clients. Fisher worked on campaigns for BMW and The Number at WCRS while Brim has produced work for Virgin Mobile and Carlsberg lager at RKCR/Y&R.