An implantable medical device assembly includes a mounting structure, an electrode protruding from a surface of the structure, between opposing sides thereof, and tissue-penetrating fixation tines, each extending from a corresponding shoulder of the structure surface, adjacent to the opposing sides. In a relaxed condition, each tine extends away from the surface and then bends toward a proximal end of the structure and back toward the surface. In an extended condition, each tine bends toward a distal end of the structure and extends along the corresponding shoulder. A holding member of a delivery tool has opposing sidewalls defining a cavity, wherein each sidewall includes a rail-like edge that fits in sliding engagement with a corresponding shoulder, to deform a corresponding tine into the extended condition, when an operator passes the assembly into the cavity. Applying a push force, to move the assembly back out form the cavity, releases the tines.