Irreversible electroporation (IRE) is a therapeutic technology for the ablation of soft tissues using electrodes to deliver intense but short electric pulses across a cell membrane, creating nanopores that lead to cell death. This phenomenon only affects the cell membrane, leaving the extracellular matrix and sensitive structures intact, making it a promising technique for the treatment many types of tumors. In this paper, we present the first in vivo study to achieve tumor regression using a translatable, clinically relevant single needle electrode for treatment administration. Numerical models of the electric field distribution for the protocol used suggest that a 1000 V/cm field threshold is sufficient to treat a tumor, and that the electric field distribution will slightly decrease if the same protocol were used on a tumor deep seated within a human breast. Tumor regression was observed in 5 out of 7 MDA-MB231 human mammary tumors orthotopically implanted in female Nu/Nu mice, with continued growth in controls.