Forsyth married Susan Clough in Cumbria in 1977 and they have three grown-up children. He is the founder of the Pimlico Tree and Preservation Trust, now the Westminster Tree Trust. In 2010 he climbed the highest mountain in Antarctica, Mount Vinson, in support of CINI and Marie Curie Cancer Care, having previously climbed Mount Aconcagua and Mount Kilimanjaro, the highest mountains in the Americas and Africa respectively. His charity fund-raising achievements are substantial and include £220,000 for DebRA for climbing Mount Kilimanjaro, £420,000 for CINI and Marie Curie Cancer Care for climbing Mount Vinson, and £500,000 to support the families of victims of 9/11 through organising a dinner in the City of London.