Background: Many health visiting services in England use the Promotional Guide system with mothers and fathers, an intervention to support their transition to parenthood, but there is little known about its use and effectiveness, especially with fathers. The aim of this study was to test the feasibility and acceptability of the Promotional Guide system with first-time fathers and pilot potential outcome measures to assess their mental health and wellbeing. Methods: A mixed methods prospective observational cohort study. Expectant first-time fathers were recruited from four London (UK) local authority boroughs. Data were collected through online pre and post intervention questionnaires, and semi-structured telephone interviews. Quantitative data were analysed using descriptive statistics and qualitative data were analysed using framework analysis. Results: Eighty-six fathers were interested in participating; 7 did not meet inclusion criteria and 79 were invited to complete the baseline questionnaire. Questionnaires completed by 45 men at both timepoints were included in the final analysis. Mean and standard deviations were calculated for all outcomes, showing a slight deterioration in the scores across all measures in the postnatal period compared to the antenatal. Ten of these men were also interviewed. Six major categories were identified: 1) Experience of health visitor contact, 2) Experience of Promotional Guides, 3) Experience of perinatal health services, 4) Experience of fatherhood, 5) Fathers’ mental health and wellbeing, and 6) Experience of the research process. While antenatal and postnatal outcomes were collected from 45 first-time fathers, none had received the intervention in its entirety. This study identified major gaps in the implementation of the Promotional Guide system with fathers. Conclusion: This study assessed recruitment of first-time fathers, time to complete recruitment, and retention rates and identified outcome measures that could be used in a future definitive study. While it wasn’t possible to examine the potential changes following the use of the Promotional Guide system, the study reported on the changes in the fathers’ ‘states’ in the antenatal and postnatal period. It provided a narrative on whether first-time fathers found it acceptable to be asked about their mental health and wellbeing, highlighted their specific needs during their transition to fatherhood, and how they wanted to be supported. It also identified barriers to implementation of the Promotional Guide system by health visitors, which need to be addressed prior to any future research into this intervention. These findings have a number of implications for researchers, health professionals, health service managers, commissioners, policy makers and parents.