Cytokinins (CKs) are adenine-derived metabolites traditionally characterized as plant hormones, yet their origin, distribution, and functions in mammalian systems remain largely undefined. Using integrated metabolomics, microbiome, and metagenomics approaches, we provide a systematic characterization of CK occurrence and potential sources in mammals. Serum profiling across five animal species revealed consistent detection of multiple CK derivatives, with concentrations markedly lower than in plant tissue. The CK storage form, zeatin-O-glucoside, predominated in mammalian sera, followed by trans-zeatin and kinetin, indicating a CK composition distinct from that in plants. Species-specific differences, such as reduced trans-zeatin in mice and lower kinetin in humans, further suggest divergent regulatory patterns. In mice, CKs were present in vascular tissues of the kidney, heart, and liver, demonstrating systemic distribution. Dietary manipulation showed that starvation significantly reduced CK abundance in serum, colon, feces, and urine, confirming that diet is a major contributor to the mammalian CK pool. Meta-omics analysis of gut microbiomes identified CK-related genes across multiple microbial taxa, with the highest representation in human microbiomes, followed by those of mouse and pig. Germ-free mouse experiments showed substantially lower CK levels than conventionally raised counterparts, establishing a microbiome-dependent contribution. Collectively, our findings identify CKs as diet and microbiome modulated metabolites in mammals, warranting future investigation to elucidate their physiological significance in mammalian biology.