Dunn was not only an artist’s assistant, but also a secretary and friend. It was Dunn who first arranged for Rossetti to get a bank account, as Rossetti would keep his money in a drawer where he, and others, could freely help themselves. Dunn worked as Rossetti’s assistant for many years, but they quarreled and Rossetti refused to pay him. The quarrel continued until Dunn returned to his native Cornwall in 1880, leaving Rossetti’s household in chaos. Rossetti then replaced Dunn with another companion; the author Hall Caine. The lost salary remained in dispute until after Rossetti’s death. When Rossetti died Dunn was very helpful to William Rossetti in his role as executor, though he considered Dunn to be an alcoholic. Dunn eventually obtained the considerable sum of money that was owed to him from Rossetti’s estate.
At the end of his life he was taken in by the critic and poet Theodore Watts-Dunton who had earlier saved Swinburne from alcoholism. The three of them lived together at Watts-Dunton’s house until Dunn died in 1899. Dunn’s watercolours and his other paintings are in collections in Cornwall and other British galleries.