<p>The SrFe_{12 − 2x}(CoSn)_xO₁₉ (x = 0.1 and 0.3) hexaferrites and their [y(SrFe_{12 − 2x}Co_xSn_xO₁₉)+(1 − y)PLA] (y = 0.0–1.0) composites were synthesized via solid-state reaction assisted by high-energy milling and melt-compression. X-ray diffraction (XRD) and Rietveld refinement confirmed the magnetoplumbite structure with minor α-Fe₂O₃ and CoFe₂O₄ phases (≤ 14 wt%) at x = 0.3. Lattice parameters expanded slightly (<i>a</i> = 5.884 to 5.892 Å; <i>c</i> = 23.06 to 23.09 Å), accompanied by crystallite growth (<i>τ</i> ≈ 57.5→61.3&#xa0;nm) and microstrain increase (ε = 0.09→0.12%). Saturation magnetization (<i>M</i>s) varied nonlinearly with doping, decreasing to 216 × 10³ A·m⁻¹ at x = 0.1, then rising to 259 × 10³ A·m⁻¹ for x = 0.3, while coercivity (<i>H</i>c) dropped from 204 × 10³ to 65 × 10³ A·m⁻¹. The effective anisotropy constant (<i>K</i>_eff) ranged 2.3–3.2 × 10⁵ J·m⁻³ and the anisotropy field (<i>H</i>a) 1.46–1.67 × 10⁶ A·m⁻¹. For SrMCoSn/PLA composites, XRD revealed coexistence of amorphous PLA and hexaferrite phases, with ferrimagnetic response emerging from y ≥ 0.1. <i>M</i>ₛ increased from 3.4 × 10³ to 59.7 × 10³ A·m⁻¹ as y rose to 0.6, whereas <i>H</i>c remained nearly constant (~ 2 × 10⁵ A·m⁻¹) up to y = 0.5, decreasing thereafter. The linear rise of <i>H</i>a (2.45 to 2.60 × 10⁷ A·m⁻¹) with ferrite fraction highlights the interfacial magnetic coupling within the polymer matrix. These results demonstrate tunable structural and magnetic properties in Co-Sn-doped SrM/PLA composites, enabling their application in magnetically responsive and multifunctional materials.</p>