On 28 September 1969, Hartnett was again selected at left wing-back when Cork faced Derry in the All-Ireland final. He claimed a second successive All-Ireland medal after the 2-07 to 0–11 victory. Hartnett immediately progressed onto the Cork under-21 team and was included on the panel for the 1970 Munster Championship. He made his debut in the grade on 15 July 1970 when he lined out at left wing-back in a 2–13 to 0–10 defeat of Kerry. On 23 August 1970, he won a Munster Championship medal following Cork's 5–12 to 1–07 victory over Clare in the final. On 4 October 1970, Hartnett lined out at left wing-back when Cork qualified to play Fermanagh in the All-Ireland final. He ended the game with a winners' medal after the 2–11 to 0–09 victory. On 12 August 1971, Hartnett claimed a second successive Munster Championship medal after Cork's 1–10 to 2–05 defeat of Waterford in the final. He was again selected at left wing-back when Cork qualified to play Fermanagh in a second successive All-Ireland final. Hartnett collected a second successive winners' medal after the 3–10 to 0–03 victory. Hartnett lined out in a third successive Munster final on 22 August 1972. He ended the game on the losing side after the 1–11 to 2–07 defeat by Kerry in what was his last game in the grade. Senior Hartnett was just out of the minor grade when he was added to the Cork senior team during the 1969–70 National League. On 26 July 1970, he was an unused substitute when Cork suffered a 2–22 to 2–09 defeat by Kerry in the Munster final. On 18 July 1971, Hartnett was selected amongst the substitutes when Cork qualified to play Kerry in the Munster final. He was introduced as a 74th-minute substitute for Kevin Jer O'Sullivan at right wing-back and claimed his first winners' medal after the 0–25 to 0–14 victory. Hartnett was selected as a substitute when Cork faced Kerry in the Munster final on 16 July 1972. He remained on the bench throughout the game and ended on the losing side after a 2–21 to 2–15 defeat. On 15 July 1973, Hartnett made his first championship start when he was selected at left wing-back when Cork faced Kerry in the Munster final. He ended the game with a second winners' medal following the 5–12 to 1–15 victory. Hartnett retained his position on