Eva Müller, he younger daughter of William Müller, a wealthy German businessman, and an English mother, Maria Henrietta, was born in 1852. Eva spent her early life in Valparaíso, Chile, and was educated by a governess and at school in London. The family lived in Portland Place.
Her mother had progressive political views and with her encouragement she worked with Octavia Hill in Marylebone, where her duties were to collect rents and to look after the welfare of tenants. Although the family was extremely wealthy, they allowed Eva to accept a nurse training post at Brownlow Hill workhouse infirmary in Liverpool.
On 18th April 1883 she married Walter Bright McLaren (1853–1912). His mother, Priscilla Bright McLaren (1815–1906), was president of the Edinburgh National Society for Women's Suffrage. Eva's husband supported the idea of women's suffrage and they both joined the Manchester Society for Women's Suffrage in 1884.
Eva McLaren also joined Josephine Butler in her campaign against the Contagious Diseases Act. These acts had been introduced in the 1860s in an attempt to reduce venereal disease in the armed forces. Butler objected in principal to laws that only applied to women. Under the terms of these acts, the police could arrest women they believed were prostitutes and could then insist that they had a medical examination. Butler had considerable sympathy for the plight of prostitutes who she believed had been forced into this work by low earnings and unemployment.In 1885 General Election, Eva's husband, Walter Bright McLaren was elected as the Liberal Party MP for Crewe. He was one of the few members of the House of Commons who supported the idea of women getting the vote. Walter served as joint secretary of the all-party committee of the Parliamentary Supporters of the Women's Franchise Bill. Eva also joined the central committee of the National Society for Women's Suffrage.
Eva McLaren became a leading member of the Women's Liberal Federation and became its first honorary treasurer in 1887. She was active in a series of suffrage pressure groups within the Women's Liberal Associations: the Union of Practical Suffragists, the Forward Suffrage Union, and the Liberal Women's Suffrage Union. In 1903 she was the author of The History of the Women's Suffrage Movement in the Women's Liberal Federation.
Eva's biographer, Linda Walker, has pointed out: "The McLaren marriage was clearly an egalitarian one - insofar as the law would allow - and both disliked the remnants of coverture which were still the hallmarks of a wife's legal subordination. A keen feminist, Eva rejected the conventional title of Mrs Walter McLaren and was known throughout her political career as Mrs Eva McLaren". She later commented "it is more important to know who a woman is than whose wife she is".
Walter Bright McLaren died from heart failure on 29th June 1912. When the First World War broke out she resumed her nursing career, and worked at a base hospital in France during the winter of 1914–15. However, suffering from poor health, she was forced to return to England.