MacAuslan, who resigned this week after 25 years with the agency, will take up two roles at Burnett. He will assume overall responsibility for the agency's international Diageo business, an account with which he was heavily involved at JWT. He will report to the Leo Burnett European president, Richard Pinder, while also liaising with Bartle Bogle Hegarty, which handles the Johnnie Walker and Baileys Diageo brands.
MacAuslan will also have a senior management role in the London office, working closely with Bruce Haines, the group chief executive. MacAuslan's job title has yet to be confirmed.
His appointment signals Burnett's intent to build its relationship with Diageo. The network is assembling a team that will work on the drinks giant's accounts and report to MacAuslan.
MacAuslan's future at JWT, where he is the deputy chairman, had been uncertain since April, when it emerged that the agency's chief executive, Simon Bolton, was intending to alter his role.
Bob Jeffrey, the worldwide chief executive of JWT, and Micheal Maedel, its worldwide president, are understood to have been keen to keep MacAuslan in an appropriate role.
Pinder said: "People with Harry's stature and experience do not come up often. When they do, it is wise to grab them with both hands."
Haines added: "Leo Burnett is a grown-up agency for grown-up clients and in Harry we have one of the most grown-up class acts in town."
MacAuslan, 47, joined JWT as a graduate trainee. During his time at the agency, he worked across a number of high-profile accounts, including Barclays and ITV.
His departure comes during a period of change for JWT's senior management team. The agency is still searching for a managing director following Kraft Foods' decision to appoint Ros King as its global business director in the spring.
Bolton commented: "We would like to thank Harry for his long contribution to JWT and wish him the best of luck in his new role."