Mickey 'Ned' O'Sullivan was born in Kenmare, County Kerry in 1952. He was educated at the local national school before later attending the Irish-speaking Coláiste Íosagáin in Ballyvourney, County Cork. After completing his Leaving Certificate, O'Sullivan travelled to London where he studied physical education at St. Mary's University. He subsequently completed his H.Dip. in education at University College Dublin. During his tenure here O'Sullivan played on the college football team in the Sigerson Cup.

At underage level O'Sullivan won a Towns Cup title in 1968, before later winning three intermediate county championships in 1968, 1970 and 1972. While O'Sullivan cherished these victories, much more was to follow at senior level.

O'Sullivan first came to prominence on the inter-county scene as a member of the Kerry minor football team in 1970. That year he lined out in the minor provincial decider against Cork. The Kerrymen recorded a comprehensive victory, giving O'Sullivan a Munster winners' medal in the minor grade. Kerry later booked their place in the All-Ireland final against Galway. In a tense encounter both sides finished level and a replay was required. That game was also a close affair with the result remaining in doubt right up until the final whistle. In the end O'Sullivan's side were defeated by a single point as Galway took the title.

By this stage O'Sullivan had established himself on the Kerry senior team. In 1972 Kerry reached the final of the National Football League and faced Mayo at Croke Park. Kerry had a relatively easy 2–11 to 1–9 victory and O'Sullivan claimed his first national title at senior level. He was later a non-playing substitute as Kerry went on to claim the Munster title but lose the All-Ireland final to Offaly after a draw and a replay.

Two more National League titles followed for O'Sullivan in 1973 and 1974, however, archrivals Cork claimed the Munster title in both years.

O'Sullivan also lined out with Munster in the inter-provincial series of games and enjoyed much success. He was first called up for Munster duty in 1975 and lined out at left wing-forward in the final against Ulster. Munster went on a goal-scoring spree that day and secured a remarkable 6–7 to 0–15 victory. It was O'Sullivan's first Railway Cup winners' medal.

In 1976 O'Sullivan was appointed captain of Munster for the inter-pro series. A defeat of Ulster set up a clash with Leinster in the final. Once again, the southern province proved too strong and a 2–15 to 2–8 score line gave O'Sullivan a second consecutive Railway Cup medal.

O'Sullivan married Marian King in 1977 and together they have two children – Éamonn (born 1979) and Bryan born (1982).

O'Sullivan was a non-playing substitute as Kerry won another set of Munster and All-Ireland titles again in 1978. He opted out of the game again in 1979 but returned in 1980 to win another pair of Munster and All-Ireland medals as a non-playing substitute once again. O'Sullivan retired from inter-county football at the end of 1980.

In 1987 O'Sullivan was entering the twilight of his club football career. Nevertheless he was still a key member of the Kenmare team that reached a third county final. Dr. Crokes of Killarney were the opponents. Kenmare triumphed and O'Sullivan added a second and final county winners' medal to his collection.

In late 1989 O'Sullivan had the unenviable task of succeeding the great Mick O'Dwyer as manager of the Kerry senior football side. Not only that but he also took charge of the Kerry under-21 football team. While under O'Dwyer, Kerry had won eleven provincial titles in twelve years. Cork were the dominant force in the province as they had claimed three Munster titles in succession between 1987 and 1989.

In 1990 O'Sullivan guided Kerry to a 25th consecutive provincial final showdown with Cork. In doing so he had the chance to stop the 'Rebels' from completing a record four-in-a-row in Munster. The game turned into a disaster from a Kerry viewpoint. An embarrassing 2–23 to 1–11 scoreline resulted in one of Kerry's biggest ever defeats at the hands of Cork. O'Sullivan's under-21 team fared much better in their championship. A 2–9 to 0–9 defeat of Cork gave Kerry the provincial under-21 title after a one-year absence. An All-Ireland final showdown with Tyrone soon followed. A high-scoring game developed over the course of the hour, however, Kerry had far too much firepower in terms of goal-scoring ability. A 5–12 to 2–11 scoreline gave Kerry the win and a first All-Ireland under-21 title as manager to O'Sullivan.

In 1991 O'Sullivan's senior footballers bounced back after a five-year provincial drought. A narrow 0–23 to 3–12 defeat of Limerick gave Kerry the Munster title for the first time since 1986 and restored some respect to the county team. While in other years Kerry would have forged ahead and contested the All-Ireland decider, a two-point defeat by Down in the semi-final saw O'Sullivan's side exit the championship at the penultimate stage. The dual manager enjoyed a bumper season as the Kerry under-21 team retained their provincial title with a narrow 1–8 to 0–10 defeat of Cork. There would be no fairytale ending, however, as the Kerry under-21 team were trounced by Tyrone in the All-Ireland decider.

1992 saw O'Sullivan manage Kerry for the final time. While the Kerry under-21 footballers claimed a third Munster title in a row, they were later beaten in the All-Ireland semi-final. O'Sullivan's senior footballers also reached a third consecutive provincial decider, this time as reigning champions. Minnows Clare provided the opposition as Kerry were installed as the red-hot favourites to retain the title. The form book was torn up, however, as Clare emerged victorious on a scoreline of 2–10 to 0–12. It was their second provincial title and a first since 1917. Following this defeat O'Sullivan resigned as manager of both the Kerry senior and under-21 teams.

In October 2005 O'Sullivan took over as manager of the Limerick senior football team.

A third season in charge in 2008 saw the Limerick team improve. A Munster quarter-final win over Tipperary set up a semi-final meeting with Cork. O'Sullivan's side were minutes away from a provincial final as Cork trailed by three points with very little time remaining. The Leesiders somehow grabbed two quick goals which changed the game completely and resulted in another narrow Limerick loss. A remarkable 4–12 to 4–3 trouncing of Meath in the qualifiers gave Limerick the chance to advance to the second round of the series where Kildare were next up. Limerick were defeated in a close game as Kildare won by 1–11 to 0–11.

2009 saw O'Sullivan enjoy his best season as manager of the Limerick senior footballers. After a disappointing league campaign championship, victories arrived over a fancied Tipperary side and Clare which meant Limerick qualified for a first Munster final since 2005. Cork stood in Limerick's way of a historic Munster championship win. Limerick stifled 'the Rebels' from the very start. For a time it looked as if Limerick would claim a first provincial title since 1896 but Cork, the red-hot favourites, scored key goals which saw them claim a narrow 2–6 to 0–11 victory. O'Sullivan's players took heart from this and entered the qualifiers at the final stage. A win in this game over Meath would have allowed Limerick enter the All-Ireland series for the first time in over a century. That game proved to be a difficult assignment. A narrow 1–13 to 2–9 victory for Meath saw Limerick exit the championship, however, it was a season to be proud for the Limerick footballers.

On 19 August 2010 O'Sullivan resigned as manager.

On 12 July 2011 O'Sullivan was named manager of the Kerry minor team.

O'Sullivan's second season in charge was an unhappy one. The team's opening game in the Munster championship quarter-final resulted in a 2–14 to 0–7 trouncing by Cork. The qualifiers beckoned for Limerick once again. Louth provided the opposition, however, for the second year in succession O'Sullivan's side were beaten by a single point and exited the All-Ireland hunt.