Millstreet Hartnett joined the
Millstreet club at a young and played in all grades at juvenile and underage levels. He enjoyed a hugely successful underage career, winning five successive Duhallow Minor Championship medals between 1965 and 1969. On 29 November 1968, Hartnett was still just 17-years-old when he lined out at left wing-back for the Millstreet senior team in the Tadhg Crowley Cup final. He ended the game with his first silverware at senior level after a 2–05 to 0–07 defeat of
St. Finbarr's.
Cork Minor and under-21 Hartnett first played for
Cork as a member of the minor team during the
1968 Munster Championship. He made his first appearance for the team on 5 July 1968 when he lined out at left wing-back in a 3–08 to 1–06 defeat of
Waterford. On 14 July 1968, Hartnett won a
Munster Championship when he again lined out at left wing-back in Cork's 2–13 to 0–02 defeat of
Kerry in the final. He won an
All-Ireland medal on 22 September 1968 following Cork's 3-05 to 1–10 defeat of
Sligo in the
All-Ireland final. On 20 July 1969, Hartnett lined out in a second successive Munster final against Kerry. He ended the game with a second winners' medal after the 3–11 to 0–12 victory. 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 the starting fifteen for the
All-Ireland final against
Galway on 23 September 1973. He ended the game with a
winners' medal as Cork claimed the title for the first time since
1945. On 14 July 1974, Hartnett participated in his fifth successive
Munster final when he was selected in his customary position of left wing-back against Kerry. He ended the game with a third winners' medal after the 1–11 to 0–07 victory. Hartnett made his sixth successive
Munster final appearance on 13 July 1975. Lining out once again at left wing-back, he ended the game on the losing side after the 1–14 to 0–07 defeat. Hartnett made no appearances for Cork during the
1975–76 National League after effectively being dropped from the team. He made several attempts to regain his place but was not included on the panel for the
1976 Munster Championship.
Munster Hartnett was selected for the
Munster inter-provincial team in advance of the
1974 Railway Cup. He was an unused substitute when Munster suffered a 1–07 to 1–04 defeat by the
Combined Universities on 26 January 1974. ==Honours==