The measurement of contaminant elements, especially cadmium and lead, in biological samples typically requires time-consuming sample preparation methods that consume organic solvents that are harmful to humans and the environment. Therefore, a fast, easy, low-cost, and green method is necessary for their determination. In this work, for the first time, the cloud point extraction method has been transferred into a simple plastic syringe, resulting in the in-syringe cloud point extraction (IS-CPE) method. The effective parameters on IS-CPE were optimized using an experimental design approach, and the optimized method was used to measure cadmium and lead in human hair samples using inductively coupled plasma-optical emission spectroscopy (ICP OES). The IS-CPE method is based on forming a hydrophobic complex of lead and cadmium ions with 8-hydroxy-quinoline (8-HQ), followed by extraction into the micellar environment of the cationic surfactant cetyltrimethylammonium bromide (CTAB), and then injection of the extractive phase of the ethanol solvent into the instrument. Using factorial design in the parameter screening stage showed that the three parameters of ligand concentration, potassium iodide (KI) salt as a clouding agent, and pH are effective at the 95% confidence level. Then, the central composite design was used in the optimization stage to obtain the response level and identify the critical values. Under optimal conditions (pH = 4.5, 8-HQ = 0.002 mol L⁻¹, and KI = 0.2 mol L⁻¹), a linear range of 10.0–1500.0 and 200.0–15 000.0 μg L⁻¹ was obtained for cadmium and lead, respectively. The detection limit of 3.0 and 62.0 μg L⁻¹ was obtained for cadmium and lead, respectively. A relative standard deviation of 3.1–4.1% at concentration 0.3–0.7 mg L⁻¹ and 5.0–5.8% at concentration 3.0–7.0 mg L⁻¹, respectively, for cadmium and lead was obtained. The proposed IS-CPE method was successfully used to measure lead and cadmium ions in human hair samples.