She was born in Dublin on 15 December (N.S.), 1732, the daughter of Thomas Garrett, a Dublin merchant of Dutch birth whose diary became a source for early Irish Methodism. He became indebted to a man named King and was threatened with imprisonment unless he gave the hand of his 'beautiful' daughter, then aged 16, in marriage to his creditor. The marriage failed and having endured ill-treatment she returned to her father's home. Both daughter and father came under the influence of George Whitefield during his second visit to Dublin in 1757 and the following year Dorothea joined the Methodist society and in 1762, during a revival, received 'a baptism of the Holy Spirit in sanctifying power', the effect of which remained with her for the rest of her life. An attractive and cultured personality, she showed great benevolence to those in need. Her husband died in 1774 and her father in 1776.
On 31 August 1784 she married the preacher John Johnson whose third wife had died the previous year and together they moved to Lisburn where he was living in retirement. John Wesley, with whom she corresponded frequently, came to regard her arrival in Lisburn as a providential strengthening of the hand of Henrietta Gayer in the increasingly important work there, particularly as Hester Ann Rogers had simultaneously reached Dublin in time to take over her Dorothea's Dublin classes. After John Johnson's death in 1803 she continued to lead several classes and bands. Her only daughter and two grandchildren predeceased her. She died of typhus fever in Lisburn on 23 July 1817.