Optical coherence elastography (OCE) is an emerging imaging technique that probes microscale mechanical contrast in tissues with the potential to differentiate healthy and malignant tissues. However, conventional OCE techniques are limited to imaging the first 1 to 2 mm of tissue in depth. We demonstrate, for the first time, OCE measurements deep within human tissues using needle OCE, extending the potential of OCE as a surgical guidance tool. We use needle OCE to detect tissue interfaces based on mechanical contrast in both normal and malignant breast tissues in freshly excised human mastectomy samples, as validated against histopathology. Further, we demonstrate the feasibility of in situ measurements >4 cm from the tissue surface using ultrasound guidance of the OCE needle probe. With further refinement, our method may potentially aid in accurate detection of the boundary of the tumor to help ensure full removal of all malignant tissues, which is critical to the success of breast-conserving surgery.