The duo will replace Nick Bell, who left in February to join J. Walter Thompson.
Following Burnett's merger last year with its sister Publicis shop D'Arcy, it was decided the role of executive creative director had become too substantial to be carried out by just one individual.
The agency's management now hopes Shearer's and Thornton's appointments will give clients a clear indication of Burnett's future direction. "We will be about creativity, collaboration and integrity," Barry Cook, the agency's chairman and managing director, said.
Shearer began his career at Partners BDDH in 1988, where he worked on accounts including BT and Honda. Subsequent moves to Simons Palmer Denton Clemmow Johnson and Bartle Bogle Hegarty were followed by his appointment as Euro RSCG Wnek Gosper's deputy creative director in 1998. Two years later, he took sole charge of the creative department at Wieden & Kennedy Amsterdam.
Thornton worked at JWT as a copywriter for ten years before becoming the creative director of TBWA/GGT in 1996. Three years later, he moved to Mother as a creative director, where he worked alongside Robert Saville and Mark Waites. During his time with the agency, Thornton has worked on the COI Communications, Pimm's, ITV Digital, The Observer and Typhoo accounts.
Both Thornton and Shearer have amassed a number of creative accolades during their careers. Shearer has eight D&AD Pencils and two Cannes Lions, while Thornton has two D&AD Pencils and a BTAA gold, among others.
Cook said: "First and foremost, Jim and Paul are talented. You have only got to look at their reels to see that. Secondly, they are British. We wanted people with experience of this market."
Thornton said: "It was going to take a very, very big opportunity to make me leave an agency as great as Mother and this is a very, very big opportunity. We are very excited about building on what Nick has started."
Shearer added: "There is a new generation of creative directors in London and we have the opportunity to be part of that. Burnett is an agency that has proved that, despite its size, it can still do good work."
The appointments follow the recent decision to move the Leo Burnett Group out of Sloane Street into D'Arcy's old offices in Kensington Village.
The move was designed to allow the main agency to forge closer working relationships with fellow members of the group, such as Arc and Masius.