John started in advertising as a junior Art Director at Benton and Bowles, London in 1965.
In 1967 he joined the Cramer Saatchi consultancy which became Saatchi & Saatchi in 1970, where he was a founding shareholder. One year later he was appointed Deputy Creative Director. John left in 1973 to co-found TBWA, London as Creative Director. The agency was the first to be voted Campaign Agency of the Year in 1980.
In 1982, John left to start Bartle Bogle Hegarty. Four years later, in 1986, BBH was voted Campaign magazine's Agency of the Year and won the title again in 1993, 2003, 2004 and 2005.
John was awarded the D&AD President's Award for outstanding achievement in the advertising industry. In May 2005, the International Clio Awards awarded John with the Life Time Achievement award for his outstanding achievement in the industry. In 2007, John received a knighthood in the Queen’s birthday honours. John has also been appointed to the New York’s One Club Hall of Fame and in 2008 was inducted into the New York Art Directors Club Hall of Fame.
In 2011, John launched his first book, “Hegarty on Advertising” to critical acclaim. In the same year he was the first person to be awarded Cannes Lion of St Mark award for his outstanding contribution to creativity.