Thomson came from a footballing family: his father Jim and brother Gary were both Balmain players. His uncle, Allan Thomson was an Australian international. Ian played President's Cup for Balmain in 1974. He then spent two seasons with Queanbeyan under coach Don Furner, earning representative honours with Country Firsts in 1976.[3] He returned to Sydney, signing with Manly-Warringah for the 1977 NSWRFL season. The following year he represented New South Wales and was first selected to play for Australia in the first test against New Zealand. The 1978 NSWRFL season's Grand Final was to be played by the Manly-Warringah and Cronulla-Sutherland clubs. The game ended in a draw and resulted in a re-play which was won by Manly-Warringah, with Thomson playing in both matches. After that he went on the 1978 Kangaroo tour.
In the second-last round of the 1980 regular season, Thomson was sent off for a high tackle and was subsequently suspended by the judiciary for the rest of the season.[4]
In November 1980 Thomson agreed to play for two seasons with the Balmain Tigers,[5] however he played only one season.[6] The following year he returned to the Sea Eagles and reached the Grand Final which was eventually lost to the Parramatta Eels.
In 1983 Thomson broke his arm in the first minute of a mid-season match against Eastern Suburbs[7] which resulted in a further break, four operations and the insertion of a steel plate, in 1984 Ian mode to England Rugby Club, Swinton Lions, where he represented Swinton Lions in the position of prop forward, playing 12 times, before moving back to Australia.
eventually forcing his retirement in 1986. He then started coaching in the Newcastle area.