He played football with his local club St Vincent's and was a senior member of the Dublin county team from 1974 until 1985.
Mullins subsequently returned to Dublin where he secured a teaching job in Greendale Community School in Kilbarrack on the north side of the city.
In 1980 he took eighteen months leave of absence from Greendale to complete a Master of Arts in athletics administration at New York University.
That year he was appointed Head Teacher of Carndonagh Community School in Inishowen, County Donegal, Ireland.
[9] St Vincent's lost out to UCD in the next two county finals; however, both sides met in the championship decider for a fourth consecutive year in 1975.
Mullins's side were awarded the title after the collegians gave them a walkover due to the final clashing with the university exams.
[11] Mullins picked up a third county winners' medal in 1976, as St Vincent's retained their title after a narrow 3–12 to 1–15 victory over UCD.
In spite of some great displays in the county champions, St Vincent's were later defeated by Summerhill of Meath in the Leinster final.
Na Fianna provided the opposition, however, St. Vincent's engineered a win and Mullins added a fifth winners' medal to his collection in that competition.
The men from the west, who had been beaten in two of the previous three championship deciders, took a 1–4 to 0–5 lead at half-time, however, the real turning point of the game came in the 52nd minute.
On a rain-soaked day John Egan and substitute Ger O'Driscoll scored two goals for Kerry and 'the Dubs' were ambushed by 2–12 to 0–11.
Immediately after the game started he careered through the Kerry half-back and full-back lines, however, his shot at goal went wide.
[17] The eleven-point victory in this game made Mullins's side the favourites to secure a remarkable third All-Ireland title in-a-row.
A free was awarded and the Kerry forward lobbed the ball over the head of Paddy Cullen, who was caught off his line arguing with the referee.
[21] Dublin and Mullins secured an historic sixth Leinster title in 1979 after a narrow 1–8 to 0–9 victory over a resurgent Offaly team.
[17] A record sixth consecutive All-Ireland final appearance soon followed, with Kerry providing the opposition for the fourth time in those six years.
The midfielder was lucky to escape with his life, however, it now looked as if the injuries which he sustained would signal the end of his football career.
In 1983 Mullins was back as a midfield marshal as 'the Dubs' took on reigning All-Ireland champions Offaly in the Leinster final.
A five-point victory on that occasion gave him a seventh provincial winners' medal, a truly remarkable achievement after his accident.
[17] A thrilling draw and a replay against Cork allowed Dublin to advance to an All-Ireland final meeting with Galway.
Barney Rock scored an opportunist goal ten minutes into the game when a Galway kick-out found him unmarked.
Ray Hazley, Kieran Duff and Mullins were all sent-off for Dublin while Galway's Tomás Tierney was also dismissed.
In spite of this Dublin's 'twelve apostles' hung on to secure a 1–10 to 1–8 victory and a fourth and final All-Ireland winners' medal for Mullins.
[24] Dublin, as reigning champions, entered the centenary year championship of 1984 as favourites to retain the All-Ireland title.
This favourites tag was justified as Mullins added an eighth Leinster title to his collection before later lining out in the All-Ireland decider.
[17] Kerry, a team that had carved out a reputation as one of the greatest of all-time over the previous decade, provided the opposition and a great game was expected.
[17] A ninth All-Ireland final appearance beckoned for Mullins, with Kerry providing the opposition for a second consecutive year.
Mullins retired from inter-county football in 1985, and became the caretaker manager for Dublin in 1986, along with Sean Doherty (GAA player) and Robbie Kelleher.
He had withdrawn his name from consideration on the previous occasion when the post was vacant in 2004 after a breakdown in negotiations with the Dublin County Board.
Mullins gave an exclusive interview to Gary Moran of RTÉ Sport for the Sunday Sport programme of Sunday 21 November 2004 in which he explained his decision, including the board's failure to match his ambition for Dublin football and the fact that he had no communication from the County Board for over three months after attending an interview for the post, a period during which the post was offered to at least two other high-profile figures, including Mick O'Dwyer.