McLennan began his career in Sydney, working for Australia's largest advertising firm George Patterson Bates, training in every area of the agency before pursuing a career in Account Management. In 1994, he moved to Bates Hong Kong as the International Client Services Director. He relocated to the agency's Sydney office before being appointed as the managing director of George Patterson Bates Melbourne, and in 1999 he was named National managing director. Three years later, in 2002, he moved to Young & Rubicam as chairman and CEO of Young & Rubicam Australia/New Zealand. The agency embarked on a cost-cutting and new business drive that saw it lead all agencies for two consecutive years in terms of new business revenue conversion. At the same time, the agency claimed a number of Cannes Grand Prix awards and built a market-leading digital practice upon their new client Telstra – Australia's largest telecommunications carrier. He then led the acquisition of his former agency to form George Patterson Y&R, the marketing communications group. In 2006, WPP's founder Martin Sorrell appointed him chairman and CEO of Young & Rubicam globally. Under his leadership, the firm won global clients like LG, Bacardi, Dell, Goldman Sachs and Virgin Atlantic. For more than a year McLennan worked with News Corporation executive chair Rupert Murdoch as executive vice president, Office of the chairman, News Corporation. In March 2013 McLennan was appointed as CEO and managing director of Ten Network Holdings, replacing James Warburton. McLennan reshaped the audience strategy and revitalized the network's fortunes in a short period of time by implementing new sports and reality TV programming. In 2015, McLennan stepped down from Ten Network Holdings as CEO and executive chair ahead of a corporate transaction. In 2018, McLennan acquired a number of data and digital businesses at REA Group as part of a strategy to accelerate revenue growth. In 2019, Hamish McLennan was named Chairman of HT&E, operator of ARN, Australia's leading radio network. Its brands include the KIIS Network, Pure Gold Network, iHeartRadio, and The Edge96 In June 2019, Hamish McLennan was named deputy chair of Magellan Asset Management – a leading investment management firm in Australia with $104 billion under management.
McLennan has been a participant at the World Economic Forum in Davos, Switzerland, where he has served on panels, ranging from the future of the automotive industry to innovation. He also participated in the Davos Reverse Mentoring Programme, designed to increase the visibility of female employees in media companies. Since 2007, McLennan has curated seminars at the Cannes Lions International Festival of Creativity featuring speakers such as Rupert Murdoch, Peter Chernin, Roger Daltrey, and Harvey Goldsmith.