Simon is recognised nationally and internationally as a leader in the field of environmental law. After initially practicing as a solicitor (from 1978) with Blake & Riggall (now Ashursts), Simon became a barrister in Victoria in 1984 and was appointed a Queen’s Counsel in 1995, specialising in environmental, planning, heritage, natural resource and climate change law.

Since June 2013, Simon has been a Vice Chancellor’s Professorial Fellow at Monash University. Previously Simon had been Adjunct Professor of La Trobe University’s La Trobe Institute for Social and Environmental Sustainability (2010-2013). In 2006 Monash University awarded Simon its Distinguished Alumni Award for 'outstanding professional achievement, inspirational leadership and exceptional human qualities' essentially for his commitment to the environment.

In 1985 Simon was a founder of what became the Environment Institute of Australia, being a member of the inaugural Steering Committee to explore options to establish the Institute. From preparing the first draft of its initial Constitution, Simon became its Public Officer upon incorporation, remaining in that office for 15 years, until it became the peak institute of practitioners whose core professional discipline is environmental management. Simon became the Institute’s longest-standing president. During his 10 year presidency from 1995 to 2005, he guided its expansion into New Zealand in 2002, it thus becoming the Environment Institute of Australia and New Zealand. He also oversaw the creation of the Certified Environmental Practitioner Scheme during his presidency, becoming a foundation member of the CEnvP. Board for its first years. He remains actively involved with the EIANZ, in particular in his capacity since mid 2013 as Chair of the EIANZ Professional Practice Disciplinary Committee.

Apart from being appointed a Fellow of the Institute in 1998 and awarded the Institute’s highest honour, that of Honorary Life Member, in 2005, the Institute named in his honour its highest award in environmental professional practice, which is awarded annually in Australasia, the Simon Molesworth Award.

Simon was the foundation Chairman of the Executive Board of the International National Trusts Organisation (INTO) from December 2007 to September 2015. From 2005 he was an inaugural member and then Chair of the INTO Steering Committee. INTO is the coordinated voice for National Trusts and like heritage organisations worldwide with some six million members and tens of millions supporters. As Chairman of INTO, Simon led INTO’s work on Climate Change, and was INTO’s Head of Delegation to a number of the UN’s UNFCCC COP conferences.

In 1989 Simon was awarded the Australian Environmental Law Award for being a person who has 'made an outstanding contribution to the development and understanding of environmental law in Australia'. In 1994 he was appointed a Member of the Order of Australia for his 'services to the community in the field of conservation and the environment in particular through the National Trust' and in 1995 he was awarded the State Environment Award by the Government of Victoria in recognition of his 'significant contribution to environment protection in Victoria 1970-1995'. In 2003 the Australian Government awarded him the Centenary Medal 'for service to the community through heritage and the environment' and the United Nations Association of Australia awarded him the 2003 World Environment Day Award for Outstanding Service to the Environment. In June 2012 Simon was elevated within the Order of Australia by being appointed an Officer of the Order of Australia for 'distinguished service to conservation and the environment, to heritage preservation at national and international levels, to the professions and natural resource sectors, and to community health organisations'. In 2013, the Law Council of Australia, Legal Practice Section, awarded Simon its Lifetime Dedicated Service Award for 'Lifetime of dedicated service to the conservation of natural and cultural heritage, to natural resource management and to community health'.