This study sought to investigate the influence of family background on English language proficiency among learners in Nine and Twelve Years Basic Education (YBE) from selected schools in Musanze district, Rwanda. The study adopted the convergent parallel design, utilizing a mixed approach. The study was carried out in 10 selected schools in different sectors within Musanze district. The population of the study counted 9321 subjects, from which a sample of 384 individuals was selected, comprising 225 students, 100 parents, and 59 teachers. The sample size was determined using Yamane’s formula. Random and purposive sampling techniques were used in selecting the involved participants. The data were collected using a structured questionnaire (with 5-point Likert scales), an interview guide, and a document review guide. The data were analyzed using percentages, frequencies, means, standard deviation, and regression analysis. The findings revealed that parents’ and siblings’ educational levels have a positive and significant influence on English language proficiency among learners in 9 and 12 YBE schools in the Musanze district of Rwanda (standard beta coefficient =.972; p-value =.000<0.05). It was found that family location has a positive and significant influence on English language proficiency among learners in 9 and 12 YBE schools in the Musanze district of Rwanda (standard beta coefficient =.981; p-value =.001< 0.05). It was found that family economic status has a positive and significant influence on English language proficiency among learners in 9 and 12 YBE schools in the Musanze district of Rwanda (standard beta coefficient =.983; p-value =.002< 0.05). It was also found that family language of communication has a positive and significant influence on English language proficiency among learners in 9 and 12 YBE schools in the Musanze district of Rwanda (standard beta coefficient =.985; p-value =.000<0.05). The study recommended that the Ministry of Education should implement comprehensive language programs, provide regular professional development for teachers, foster a language-rich environment, and utilize technology for language learning as a means to create an environment that promotes English language proficiency and support students in developing their English proficiency.