Background: Given the increasing complexity of acute care settings, high patient acuity and demanding workloads, new graduate nurses continue to require greater levels of support to manage rising patient clinical care needs. Little is known about how change in new graduate nurses' satisfaction with clinical supervision and the practice environment impacts on their transitioning experience and expectations during first year of practice. This study aimed to examine change in new graduate nurses' perceptions over the 12-month Transitional Support Program, and identify how organizational factors and elements of clinical supervision influenced their experiences. Methods: Using a convergent mixed methods design, a prospective survey with open-ended questions was administered to new graduate nurses' working in a tertiary level teaching hospital in Sydney, Australia. Nurses were surveyed at baseline (8-10 weeks) and follow-up (10-12 months) between May 2012 and August 2013. Two standardised instruments: the Manchester Clinical Supervision Scale (MCSS-26) and the Practice Environment Scale Australia (PES-AUS) were used. In addition to socio-demographic data, single -item measures were used to rate new graduate nurses' confidence, clinical capability and support received. Participants were also able to provide open-ended comments explaining their responses. Free-text responses to the open-ended questions were initially reviewed for emergent themes, then coded as either positive or negative aspects of these preliminary themes. Descriptive and inferential statistics were used to analyse the quantitative data and the qualitative data was analysed using conventional content analysis (CCA). The study was approved by the relevant Human Research Ethics Committees. Results: Eighty seven new graduate nurses completed the follow-up surveys, representing a 76% response rate. The median age was 23 years (Range: 20 to 53). No change was seen in new graduate nurses' satisfaction with clinical supervision (mean MCSS-26 scores: 73.2 versus 72.2, p = 0.503), satisfaction with the clinical practice environment (mean PES-AUS scores: 112.4 versus 110.7, p = 0.298), overall satisfaction with the transitional support program (mean: 7.6 versus 7.8, p = 0.337), satisfaction with the number of study days received, orientation days received (mean: 6.4 versus 6.6, p = 0.541), unit orientation (mean: 4.4 versus 4.8, p = 0.081), confidence levels (mean: 3.6 versus 3.5, p = 0.933) and not practising beyond personal clinical capability (mean: 3.9 versus 4.0, p = 0.629). Negative responses to the open-ended questions were associated with increasing workload, mismatch in the level of support against clinical demands and expectations. Emergent themes from qualitative data included i) orientation and Transitional Support Program as a foundation for success; and ii) developing clinical competence. Conclusions: While transitional support programs are helpful in supporting new graduate nurses in their first year of practice, there are unmet needs for clinical, social and emotional support. Understanding new graduate nurses' experiences and their unmet needs during their first year of practice will enable nurse managers, educators and nurses to better support new graduate nurses' and promote confidence and competence to practice within their scope.