Mary was born in 1826 and during her early childhood lived with her parents in the London area but exactly where I am not sure. This must have been a rather unsettled time as a few years earlier (in 1824) her parents had lost most of their money, when the family banking firm of Marsh Sibbald & Co became bankrupt. Mary's father, Arthur Cuthbert Marsh, found it difficult to recoup the family finances and Mary's mother, Anne Marsh, in an effort to support the family, started writing novels. Anne's first novel, "Two Old Men's Tales", published in 1834, was an immediate success, however, with a family of 7 children to support, they were still very short of ready cash. By 1839, in an attempt to keep their living costs to a minimum, they were living in Boulogne in France. Ann commented in her diary that this was a difficult time for all of them but for her youngest children including Mary it may not have had a such a detrimental impact as they had not been accustomed to the former privileged lifestyle of the family before the loss of the family finances.
Eventually things began to improve. Mary's father Arthur came into a small inheritance from a distant friend of the family. Her mother Anne Marsh also received an inheritance from her father James Caldwell. At the same time Anne was writing more books and this was also bringing in more money. With the family finances still rather difficult but improving, the family were able purchase the Eastbury Estate, near Watford, and they moved back to England to take up residence in early 1842.
Life at Eastbury was probably pretty good but Mary would have experienced great sadness when her brother Martin died in 1844 at the young age of only 20. In 1848 Mary's sister Francis married Lt Col Richard Henry Crofton (later Maj Gen) of the Royal Artillery. In 1853 Mary went to stay with the Croftons in Malta where Richard was stationed with the Army. It was here that Mary was to meet her future husband Commodore (and later Admiral Sir ) Leopold George Heath. The story in the family was that Mary was particularly good at painting and Leopold noticed one of her paintings when he was visiting friends. He enquired as to who had produced such an excellent painting and was immediately introduced to Mary. They were married 8 December 1853 at Malta. She started producing a family with the birth of their first child Arthur after which Mary returned to the UK, initially taking up residence at Moorhurst in Holmwood and then moving to Anstie Grange where she was to spend the rest of her life. She produced 7 children, 5 sons and 2 daughters. Of the sons three went on to have highly successful miltary careers and one became a very rich businessman.