Janet Martin was born in Wigtownshire, Scotland, probably some time between 1819 and 1825, to Hannah Martin and her husband, Andrew Martin, a farmer. On 19 September 1835 at Glasgow she married John Main. Her married name was sometimes spelt Mayne. When she arrived at Auckland, New Zealand, on the Sir Edward Paget in December 1850 she was a widow, and brought with her two children. She followed friends from Scotland who had come to New Zealand in 1842 on the Duchess of Argyle.
Prayer meetings concerning the formation of a Baptist church in Auckland were held in Janet Main's home in 1854. In the following year she became one of fifteen foundation members of the Wellesley Street Baptist Church. Church minutes make few references to the activities of women, but Janet Main was associated with the leading men of the congregation in the task of interviewing prospective female members.
On 8 October 1864 at her residence in Nelson Street, Auckland, she married a bachelor, Andrew MacKenzie Donald, who was then aged 29. Andrew Donald recorded his employment as a compositor but electoral records later referred to him as a farmer, in possession of land at Ōtāhuhu. By the end of the 1870s the Donalds had a home at Ōtāhuhu. It seems that there were no children of the marriage.
Janet and Andrew Donald were instrumental in the formation of a Baptist congregation at Ōtāhuhu, which began to meet at their house in 1878. Letters to the parent church in Auckland, written by Andrew Donald, bemoan the delay in building a Baptist chapel, promised for Ōtāhuhu. It is probable that Janet Donald played a significant part in pursuing this matter. The campaign was successful: a chapel was built there in 1879.
Janet Donald remained active in the Baptist church all her life and was the longest-surviving woman among the foundation members of the Wellesley Street Baptist Church. She died on 27 March 1892 and was buried at the Ōtara Presbyterian cemetery. She was survived by her husband, and a son from her first marriage, George Martin Main, who was a well-known journalist. An obituary, which appeared in the New Zealand Herald, recognised her standing in the community.
In his remarks at the burial service Richard Shalders, the founder of the Auckland YMCA and a leader in the Wellesley Street Baptist Church, referred to Janet Donald as 'the "mother" of the Baptist Church in Auckland'. He described her as 'noted for her devout zeal, kindliness of heart, and benevolence'. These characteristics were clearly matched by a practical vision and organising ability. Although much of her work was carried out behind the scenes, Janet Donald was one of the acknowledged leaders of the church and devoted her life to ensuring its secure establishment.