Mainelli's early research in aerospace and computer graphics at the Harvard Laboratory for Computer Graphics and Spatial Analysis led him to start Swiss companies in seismology, cartography, and energy information for Petroconsultants, an oil and gas information services company, from 1979 to 1984. He produced Mundocart, a complete 1:1,000,000 digital map of the world for use in petroleummapping in 1983, while directing the Geodatcartography project. Mainelli left Petroconsultants in 1985, and joined Arthur Andersen a year later as a senior manager. He later became a senior partner and board member of the accountancy firm BDO Binder Hamlyn from 1987 to 1995.
A Fellow and Trustee of Gresham College, Mainelli is a visiting professor of UCL's Bartlett School of Sustainable Construction, Honorary Fellow of King's College London, an Economic and Business Faculty Fellow of Goodenough College, and a former Visiting Professor of the London School of Economics. He is the current Chairman of Z/Yen Group. In 2005 Mainelli, in conjunction with Gresham College, the City of London Corporation and financial institutions, launched Long Finance's London Accord, an agreement to share environmental, social and governance research with policy makers and the public. Mainelli has lectured on new concepts of money and the role of government, publishing 28 one-hour Gresham lectures, while serving as Professor of Commerce from 2005 to 2009. The theme of his programme was "Society’s Commercial Choice – Risks and Rewards of Markets".
Mainelli was elected Sheriff of the City of London on 24 June 2019, assuming office on 27 September 2019. In view of the global disruption to public life brought about by the COVID-19 pandemic, Mainelli was re-elected, on 24 June 2020, to serve a second term as Sheriff of London for 2021.
Lord Mayor of the City of London
On 29 September 2023, Mainelli was elected Lord Mayor of the City of London, and took office on 10 November 2023, becoming the first American-born person to do so. Mainelli's mayoral theme was 'Connect To Prosper', an initiative that aimed to highlight the City of London's ability to solve global challenges. It championed the "scientific, academic and business worlds", going beyond the traditional support of finance and professional services usually associated with the role.
Writing
Mainelli has published more than 50 journal articles, 250 commercial articles and four books, including the novel Clean Business Cuisine: Now and Z/Yen, written with Ian Harris. Mainelli's economics book, The Price of Fish, also written with Ian Harris, applied his Gresham lecture series ideas to "wicked problems".
Mainelli married Elisabeth (née Reuß, 1964) in 1996. Together they undertook the restoration of the SB Lady Daphne, a Thames sailing barge built in 1923. Mainelli still supports and lectures on sailing barges and their history, having sold the SB Lady Daphne upon completing renovation in 2017. He is a committee member of the Thames Sailing Barge Match, the second oldest sailing race in the world.
Independent Publishers Gold Award for Finance, Investment and Economics (2012) - "The Price of Fish: A New Approach to Wicked Economics and Better Decisions"
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