Mimosine [β-(3‑hydroxy-4-pyridone-1-yl)-L-alanine] is a toxic non- protein amino acid, and one of its major effects is to suppress plant growth and development. Since mimosine is a compound responsible for many interesting biological activities, intensive studies have begun to be carried out on it in recent years. The current work investigated toxic effects of exogenously given 1, 2 and 3 mM L-mimosine doses on some physiological, cytogenetic, biochemical, and anatomical parameters of Allium cepa L. bulbs. For this purpose, the germination percentage (GP), root length (RL), root number (RN), and fresh weight (FW) were determinated as physiological parameters to be examined experimentally; the micronucleus (MN) frequency, chromosomal aberrations (CAs), and mitotic index (MI) were chosen as cytogenetic parameters; and the catalase (CAT) and superoxide dismutase (SOD) activities, malondialdehyde (MDA) level, and free proline (PR) content were determinated as biochemical parameters to be investigated. In addition, the changes in the root anatomical structure of the bulbs were examined under the microscope by taking cross-sections. Onion bulbs were divided into four groups as three treatments and one control (C). For seven days, the bulbs in the treatment groups were germinated with three different doses of L-mimosine (1, 2 and 3 mM), while the bulbs in the C group were germinated with tap water. Consequently, all three doses of L-mimosine caused a decrease (p < 0.05) in all investigated physiological parameter (GP, RL, RN and FW) values compared to C. Furthermore, every L-mimosine dosage resulted in a reduce (p < 0.05) in MI and an increase (p < 0.05) in MN and CA frequency compared to C group. L-mimosine application promoted CAs such as irregular mitosis, stick chromosome, lagging chromosome, protruded out chromosome, pole deviation and alignment in root meristem cells. Exposure to L-mimosine resulted in dose-related increases (p < 0.05) in SOD and CAT enzyme activity as well as MDA and PR levels compared to C group, indicating that L-mimosine also induced toxicity by causing oxidative stress in cells. Also, especially exposure to a dose of 3 mM L-mimosine caused the anatomical damages such as deformations of the epidermis and cortex cells, necrosis, buildup of certain chemical compounds in the cortex cells, thickening of the cortex cell wall, formation of vacuoles in the cell nuclei and flattened cell nuclei in the root tip meristem cells. In summary, since L-mimosine showed an inhibitory effect in the Allium cepa L. test material, it was concluded that the compound induced multi-dimensional toxicity, and the Allium test proved to be a highly valuable tool in identifying this toxicity.