In heavy quark limit, the lowest-lying charmed baryons with two light quarks can form an SU(3) triplet and sextet. The ${\Xi }_c$ in the SU(3) triplet and ${\Xi }_c^{\prime }$ in the sextet have the same $J^{PC}$ quantum number and can mix due to the finite charm quark mass and the fact the strange quark is heavier than the up/down quark. We explore the ${\Xi }_c$–${\Xi }_c^{\prime }$ mixing by calculating the two-point correlation functions of the ${\Xi }_c$ and ${\Xi }_c^{\prime }$ baryons from lattice QCD. Based on the lattice data, we adopt two independent methods to determine the mixing angle between ${\Xi }_c$ and ${\Xi }_c^{\prime }$. After making the chiral and continuum extrapolation, it is found that the mixing angle θ is ${1.2}^{\circ }\pm {0.1}^{\circ }$, which seems insufficient to account for the large SU(3) symmetry breaking effects found in weak decays of charmed baryons.