BACKGROUND: Infants are a vulnerable population whose nutrition changes as complementary foods are introduced; a process which may modify patterns of exposure to dietary mycotoxins. However, exposure monitoring of dietary mycotoxin mixtures in several biological samples obtained from breastfed and non-exclusively breastfed children is scarce. OBJECTIVES: To examine mycotoxin co-exposure patterns in infants using a multi-specimen, multi-mycotoxin approach. METHODS: Breast milk, complementary food and urine obtained from 65 infants, aged 1-18 months, in Ogun state, Nigeria, were analyzed for mycotoxins using ultra-sensitive LC-MS/MS approaches. RESULTS: Complementary food was contaminated with seven distinct classes of mycotoxins including aflatoxins (9/42 samples; range: 1.0-16.2ug/kg) and fumonisins (14/42; range: 8-167ug/kg). Aflatoxin M1 was detected in breast milk (4/22), while six other classes of mycotoxins were quantified; including dihydrocitrinone (6/22; range: 14.0-59.7ng/L) and sterigmatocystin (1/22; 1.2ng/L) detected for the first time. Mycotoxins were detected in 64/65 of the urine samples, with seven distinct classes of mycotoxins observed demonstrating ubiquitous exposure. Two aflatoxin metabolites (AFM1 and AFQ1) and FB1 were detected in 6/65, 44/65 and 17/65 samples, respectively. Mixtures of mycotoxin classes were common, including 14/42, 22/22 and 56/65 samples having 2-4, 2-6 or 2-6 mycotoxins present, for complementary food, breast milk and urine, respectively. Aflatoxin and/or fumonisin was detected in 12/14, 4/22 and 46/56 for complimentary foods, breast milk and urine, respectively. Furthermore, the detection frequency, mean concentrations and occurrence of mixtures were typically greater in urine of non-exclusively breastfed compared to breastfed children. CONCLUSIONS: The study provides novel insights into mycotoxin co-exposures in children in a mycotoxin high-risk country without proper food safety measures. Albeit a small sample set, it highlights significant transition to higher levels of infant mycotoxin exposure as complementary foods are introduced, providing impetus to mitigate during this critical early-life period and encourage breastfeeding.