She was a co-founder and a member of the board of directors of the Svensk sjuksköterskeförening ('Swedish Nurses Association') or SSF in 1910 and chairperson in 1914–1933.

In 1921, she became one of the first five women to be elected to the Swedish Parliament after women suffrage alongside Nelly Thüring (Social Democrat), Agda Östlund (Social Democrat) and Elisabeth Tamm (liberal) in the Lower chamber, and Kerstin Hesselgren in the Upper chamber.

As an MP, Bertha Wellin was foremost engaged in issues regarding the nursing profession.

As a conservative, her views of nursing was that it was not to be regarded as a profession but as a holy calling of mercy, and her work both as an MP and as head of the Nursing Association was affected by this view, which blocked questions regarding higher wages and less working hours.

This caused gradually more conflicts within the nursing association when the association became more dominated by nurses who did not come from a wealthy background, needed their wages for financial reasons, demanded higher wages and set working hours and wished to be regarded as professionals rather than philanthropic workers.