Montelius was the daughter of the government defence minister and noble General-Lieutenant Alexander Reuterskiöld and Anna Schenström. She was educated at the fashionable Girl School Hammarstedtska flickskolan in Stockholm, and married professor Oscar Montelius on 20 September 1871. She is described as diminutive, calm, kind and thoughtful, dutiful and always busy with her many projects. She had a bad eyesight and eventually became blind in one eye. Her own personal ideals was simple and strict. Montelius was regarded as a central figure and an ideal among the women of the higher middle class and upper burger classes in Stockholm, and Lydia Wahlström often engaged her as a so-called exam witness for the students of the Girl college Åhlinska skolan.
Agda Montelius was the leading figure of the Swedish philanthropy in the early 20th century. Her principle was to help people help themselves.
She was member of the comity (1885–1901) and the chairperson (1900–01) of the literary society Nya Idun (New Idun); the Maria skyddsförening (Maria Protection Society) 1879–92, Co-founder and chairperson of Föreningen för välgörenhetens ordnande or FVO (Society of Organised Charity) in 1889–1911 as well as managing director of the FVO central committee in 1911–1920. She was a member of the central comity in the Sällskapet för uppmuntran av öm och sedlig modersvård (Society for the Encouragement of Tender and Decent Motherly Care) in 1901–20, co-founder and comity member of the Centralförbundet för socialt arbete (Central Comity of Social Work) or CSA in 1903-09 and Svenska fattigvårdsförbundet (Swedish Poorcare Society) in 1909–20.
Through her philanthropic work, she also became involved with the work for women's rights. She was a supporter of Difference feminism and believed it to be important for women to participate in politics and the organization and formation of society on order to protect the rights of the sick, the weak and needing and to make society a home.
In 1886, Agda Montelius officially became a member of the women's rights organisation of Sophie Adlersparre: Fredrika Bremer Association or FBF. Two years prior, she had been one of its co-founders. Formally, the FBF was headed by Hans Hildebrand because Adlersparre thought it necessary for the society to be headed by a male for it to be taken seriously. In reality, however, Adlersparre functioned as its chairperson, and upon the death of Adlersparre in 1895, she was succeeded by Agda Montelius. Montelius was initially called vice chairman, but in 1903, she formally became chairman, officially the first female chairperson of the FBF.
The goal of the FBF was to work for women's rights, but previously, it had not worked for woman suffrage. In 1899, a delegation from the FBF presented a suggestion of woman suffrage to prime minister Erik Gustaf Boström. The delegation was headed by Montelius, accompanied by Gertrud Adelborg, who had written the demand. This was the first time the Swedish women's movement themselves had officially presented a demand for suffrage.
In 1902, the Swedish suffrage movement National Association for Women's Suffrage (Landsföreningen för kvinnans politiska rösträtt, LKPR) was founded. Montelius never became a formal member, probably because of her chairmanship of the FBF, but she was informally active for the LKPR. She performed many tasks for the LKPR, she made the resources and the members of the FBF available for service in the LKPR, and she made the paper of the FBF, Dagny, the spokes organ of the LKPR until 1911. In 1911, when the LKPR abandoned its political neutrality by a resolution of boycott against political partys opposing woman suffrage, she stopped the use of the FBF:s paper Dagny as the paper of LKPR.
She was a consultant in the governmental comity for the reformation of the marriage rights law in 1912, which eventually (in 1920) led to husband and wife being given equal rights within marriage.
Agda Montelius was also active within the peace movement, during which FBF again collaborated with the LKPR. During World War I, the LKPR took the initiative for a peace organisation formed by women of the neutral countries with the aim to form pressure on the neutral governments to act as mediators between the warring parties. The Peace Movement was formed by the LKPR with members also from Fredrika-Bremer-förbundet, KFUK, the social democratic women's organisations among others, with Anna Whitlock, Emilia Broomé and Kerstin Hesselgren as leading members. A great peace manifestation was to take place 19 February 1915 organised by the Swedish women with support and participation also from the women of Denmark and Norway. The 18 February, however, Agda Montelius was called to the Queen, Victoria of Baden, who demanded a stop of "The foolish presumption of women" to involve in politics. King Gustav V of Sweden interrupted and said that women were of course entitled to present demands to the government, but that the situation made it difficult, and referred to the minister of foreign affairs, who warned them that such an action could damage the Swedish neutrality. The action is therefore silenced in both Sweden, Denmark and Norway, and the blame is by the women involved place on Victoria of Baden. The Swedish Peace Movement does, however, send 16 delegates to the international women's peace movement in the Hague in April 1915.
Agda Montelius was given Illis Quorum in 1910.