Background: Rates of bilateral mastectomy continue to rise in average-risk women with unilateral breast cancer. We aim to characterize psychosocial predictors of surgical procedure and how psychosocial outcomes change over time after surgery for breast cancer. Methods: A prospective cohort of women with unilateral, nonhereditary breast cancer were recruited at University Health Network in Toronto, Canada between 2014-2017. Women completed validated psychosocial questionnaires (BREAST-Q) pre-operatively, and 6 and 12 months after surgery. Outcomes were assessed between three surgical groups (unilateral lumpectomy, unilateral mastectomy, bilateral mastectomy). Predictors of surgical procedure were identified using a multinomial logistic regression model. Change in psychosocial scores over time according to procedure was assessed using linear mixed models. All models control for age, stage, reconstruction and treatment. P values < .05 were considered statistically significant. Results: 506 women underwent surgery as follows: 216 unilateral lumpectomy (43%), 181 unilateral mastectomy (36%) and 109 bilateral mastectomy (22%). In the multinomial regression model, younger age (p < .01), and lower chest physical (p = .03) and sexual well-being (p = .02) predicted having bilateral mastectomy over unilateral lumpectomy while younger age (p < .01) and lower disease stage (p = .02) predicted bilateral mastectomy over unilateral mastectomy. The mixed model demonstrates that breast satisfaction follows a non-linear pattern of change over time, with 6- but not 12-month scores being significantly different from baseline (p = .015). Procedure predicts baseline satisfaction (p = .016), with bilateral mastectomy having worse satisfaction than unilateral lumpectomy. Procedure also predicts change in satisfaction, with unilateral and bilateral mastectomy having lower scores across time than lumpectomy. While a significant improvement in psychological well-being is detected by 12 months (p = .02), those with unilateral and bilateral mastectomy have worse psychological well-being over time compared to lumpectomy. Women having mastectomy start with worse physical well-being than those in the lumpectomy group, but their physical well-being does not decline as much as the lumpectomy group over time (p < .01). Conclusions: Definitive surgical procedure affects the trajectory of psychosocial functioning over time. This emerging data may be used to further facilitate surgical decision-making in women considering contralateral prophylactic mastectomy.