A destructive hard running centre, Harris made 190 appearances for the Roosters and a further 33 for the Bears. A Brisbane junior, Harris was playing in Papua New Guinea for the Koni Tigers club when recruited by Eastern Suburbs. Harris was a member of some of Eastern Suburbs greatest teams, winning premierships with that club in 1974 and 1975. Harris was also a member of the 1974 and '77 pre-Season cup winning sides, the 1975 and '77 midweek cups and the Roosters side that thrashed St Helens in the first World Club Challenge ever held,in the 1976 season. Harris also won the prestigious media award 'Rookie Of The Year' in his first season, and in 1972 he won another prestigious award taking out the Sun-Herald player of the year award. Harris played out the final years of his career with the North Sydney club.
Harris trialled with the American gridiron team, the Philadelphia Eagles, in the early part of his career, In 1973 Harris played with the Canadian' Grid Iron side, Montreal Allouettes, a move that cost him a place on the Kangaroo tour, and although successful he turned his back on the American game because he was only being used in the limited role of a long range kicker.
Harris played in just the one test match for Australia, that was against New Zealand in 1972, however he represented Australia in the 1970, 1972, 1975 & 77 World cups.
Since his retrement the burly centre has taken on a role as a National Rugby League judiciary member. It was said that, during his career, there was not a more fearsome sight, in rugby league for an opposition defender than seeing a rampaging Mark Harris heading in his direction.