Cognitive changes following adjuvant treatment for breast cancer (BC) are well documented following chemotherapy. However, limited studies have examined cognitive and/or language functions in chemotherapy-naive women with BC taking tamoxifen (TAM). Using ambulatory cognitive assessment, we investigated the trajectory of cognitive and language changes during early period of adjuvant endocrine treatment (TAM) in women with BC at two time periods (pretreatment and 2 months after treatment began). Four women with BC and 18 cognitively healthy age-matched controls completed three cognitive tasks using smartphones, during a short time period (5 days) and repeated them at two time periods. To determine language ability, language samples were collected at two time periods, where the participants described two stories from two wordless picture books and samples were assessed using core lexicon analyses. Wilcoxon-signed rank tests were computed to identify differences in linguistic and cognitive performances of both the groups at two time periods. No significant within-group or between-group differences were seen on the cognitive and language tasks at the two time periods; however, women with BC performed more poorly compared to the control group. We did see decline in some women with BC and not in others, in cognition and language during initial course of TAM treatment. However, the approach we used to assess these changes is valuable and innovative. This approach will help refine current research paradigms for determining cognitive and linguistic changes and will help determine if women with BC might require language intervention in the future.