Daughter of distiller and chemist George Maclagan and Janet Colville of Stirling, she was born on the family's farm at Braehead near Denny. Her father died in 1818, as did her paternal grandfather, Frederick Maclagan, parish minister at Melrose, and her mother moved the family to Stirling, to a house in Pitt Terrace, a wealthy part of the town near the modern Stirling Council offices.
Her mother died in 1858, and until that time Christian Maclagan engaged in philanthropic activities, establishing a Sunday School and subscribing towards the cost of a library. After the Disruption of 1843 she joined the Free Church of Scotland and in 1865 she funded the building of a new kirk. Her relationship with the Free Church soured in the 1870s and she sued to reclaim the church which she then gifted to the established Church of Scotland. She apparently received a bequest from one of her brothers at around the time of her mother's death, and this established her as a woman of some wealth. Her estate was valued at 3100 pounds sterling at her death.
She was well-educated, knowing Latin, French, Greek, and Gaelic—her paternal grandfather had tried his hand at a translation of the Bible into Gaelic—well and some Italian. She was also an artist of some skill.
She devised a special method for taking rubbings from sculptured stones. Possibly her greatest contribution to posterity was her meticulous collection of rubbings of Celtic Christian crosses and Pictish symbol stones, made from c 1850 onwards
As a woman Maclagan was disbarred from obtaining a fellowship of the Society of Antiquaries of Scotland, and instead she was merely a Lady Associate. As a result of this, she sent her rubbings to the British Museum.