Background: AIDS was first diagnosed in 1983. We know that HIV and AIDS have been in Tanzania for more than three decades. Although efforts to find a cure for AIDS have not yet been fruitful, the Government of Tanzania initiated the rollout of free antiretroviral therapy (ART) in 2004. This intervention has dramatically reduced rates of mortality and morbidity and improved the quality of life for people living with HIV (PLHIV). However, the availability of ART does not guarantee that all PLHIV have equal access to it. Gender can increase a person’s vulnerability to HIV and influence his or her ability to access information about preventive measures, care, support, and treatment. The purpose of this study was to investigate how gender-related factors affect access to and follow-through of this important treatment for HIV and AIDS. Methods: We conducted our study in Njombe District of Njombe Region, Tanzania, from October to November 2015. The region has the highest HIV prevalence rate in the country. We used a cross-sectional design to collect data using a structured questionnaire administered at one public permanent care-and-treatment clinic (CTC) and one mobile CTC. The study population consisted of male and female PLHIV who were more than 18 years of age, were registered in an ART program in Njombe District for at least three months, and had consented to participate in the study. A combination of convenience and snowball sampling techniques was used to capture respondents with the desired characteristics. A total of 132 respondents (97 females and 35 males) completed the questionnaire. Five key informant interviews (KIIs) were conducted with health workers, nongovernmental organization (NGO) staff, and caretakers at the family level. Two focus group discussions (FGDs) were also conducted; one consisted of eight females, the other had seven males. In addition, secondary data from Njombe District health facilities were collected. Results: Our study found that heterosexual intercourse with a spouse was the leading cause of HIV transmission for more than two-thirds of the women (64.9%), and casual sex outside of marriage was the major cause for more than half of men (54.3%). The main reasons respondents gave for getting tested for HIV were compulsory HIV screening during prenatal visits, HIV awareness campaigns (especially for men), and unhealthy symptoms such as weight loss and frequent diseases. Most PLHIV (86% of men and 80% of women) started on ART within the same year they were diagnosed. Partners did not usually get tested for HIV together; men were typically more reluctant, and they denied the problem, usually asserting their wives were the source of the virus. Women were less likely than men to consult their spouse/partners before getting tested and less likely to get support from their partners after they were tested. Women (especially married women) were more likely to face social problems, including stigma, when disclosing their HIV status to their partners. Some women feared family quarrels and harsh consequences, such as being physically abused and/or divorced. Communication barriers during the early stages of deciding to get tested, and the poor responses women received from their partners when the women consulted them, affected women’s next phase of living with HIV and had a negative impact on ART access and adherence to services. Respondents had a working knowledge of how to adhere to ART and practice safe sex, but they rarely followed safe-sex practices. While nearly all the women (96.9%) took all their medicine, they were less likely than the men to follow the clinic schedule precisely. Women in the FGD said that heavy household workloads made it more difficult for them to take their medications on time. Conclusion: Gender inequity adversely affects adherence to ART in different ways for women and men living with HIV. This study has improved understanding of gender differences in ART access and adherence. We hope this will contribute to the development of more effective gender-based interventions that can potentially enhance ART access and follow-through.