Masson was born in Aberdeen. In the 1760s he went to work at Kew Gardens as an under-gardener. Masson was the first plant collector to be sent from Kew by the newly-appointed director Sir Joseph Banks. He sailed with James Cook on HMS Resolution to South Africa, landing in October 1772. He stayed until 1775 and sent back to England over 500 plant species. In 1776 he went to Madeira, Canary Islands, the Azores and the Antilles. In 1783 he collected plants in Portugal and in January 1786 returned to South Africa, remaining until March 1795.
Massons only book, Stapeliae Novae, on the South African succulents also known as "carrion-flowers" because of their smell, was published in 1796.
In September 1797 he set sail for North America, arriving in New York in December. In the next few years he travelled widely collecting plants and seeds, visiting Niagara and Lake Ontario. He died in Montreal in December 1805, and he was buried there at the Scotch Presbyterian Church (later known as the St. Gabriel Street Church).
The genus of plants Massonia is named after Masson. There is a commemorative plaque to Masson in Cruickshank Botanical Gardens, Old Aberdeen.