Background: Family planning metrics categorise women as those desiring to space or to limit future births, or according to their age. In Tanzania, we extended this categorisation to explore the family planning needs of women who want to delay their first birth. Aim: To investigate the characteristics, needs, sources of modern contraception and quality of care for women who want to delay their first birth; and to explore community and health provider’s perceptions about using modern contraception to delay first birth in Tanzania. Methods: In 2014, a mixed methods study was implemented in southern Tanzania. Methods included analysis of household survey interviews with women aged 13-49 years to determine practice, health facility data to assess provision of services, and in-depth interviews and focus group discussions for evidence of perceptions and acceptability. Results: From the survey, 4% of 2128 women aged 13-49 years were categorised as ‘delayers of first birth’. The majority were teenagers (82%) and unmarried (88%). About half were currently using modern contraception, predominantly injectables, accessed from public facilities. Forty one percent of the delayers had unmet need for modern contraception. Indicators of quality service provision were low for all women. From qualitative interviews, the majority of community members and health care providers said that the use of modern contraception to delay first birth was widely acceptable for students, young, unmarried and women in unstable marriage, but implants and intra-uterine devices and systems were perceived as inappropriate methods. A lack of consistency on the definition of ‘young’ had direct implications for access, autonomy, confidentiality and consent for young people. Conclusions: A small but important group of sexually active women desire to delay their first birth. These women have very high unmet needs for modern contraception. Routinely categorising and measuring delayers of first birth acknowledges their unique needs and could help to catalyse a policy and programmatic response.