In order to identify the potential benefits and challenges of implementing student portfolios as quality mathematics assessment, a pilot study was conducted with teachers in various secondary school settings. The multi-case study consisted of five teacher participants from geographically and demographically differing contexts, four in the USA and one in Canada. They were interviewed and surveyed, exploring their motivations for utilizing a portfolio system of mathematics assessment for the 2020-2021 school year, the design of their portfolio system, and resultant impacts on student dispositions around mathematics. Each participating teacher implemented different structures of portfolio assessment, including the types of items included and how the items were assessed. Results showed that compared to traditional multiple-choice tests, teachers felt portfolios were a better reflection of their students' "extra-mathematical" skills, such as problem-solving and persistence. Teacher surveys and interviews also demonstrated confidence that the use of portfolios as assessment was superior to more traditional measures in terms of adhering to good pedagogical practices. Through questionnaire data and interviews, teachers reported portfolios were particularly beneficial for students for whom English is not their primary language, as well as students with special needs. The paper lays the groundwork for additional research studies in the area of student mathematics portfolios based on the provided framework.