Jansher won the World Junior Squash Championship title in 1986. He also turned professional that year. At the time, the men's professional tour was dominated by another great Pakistani player – Jahangir Khan. Jahangir won the pair's first few encounters in late-1986 and early-1987. Jansher then scored his first win over Jahangir in September 1987, beating him in straight games in the semi-finals of the Hong Kong Open. Jansher then went on to beat Jahangir in their next eight consecutive encounters. This included a win in the semi-finals of the 1987 World Open, following which Jansher claimed his first World Open title by beating Australia's Chris Dittmar in the final.
Jansher officially announced his retirement from squash in 2001. He won a total of 99 professional titles and was ranked the World No. 1 for over 10 years.
In August 2007, Jansher announced that he was coming out of retirement to play in a Professional Squash Association tournament in London in October 2007. He said in a news conference that the reason for his comeback was that, "I feel I am mentally and physically fit to play the international circuit for another three to four years". He lost in the opening round of the event to England's Scott Handley 11–9, 6–11, 6–11 0–11.
In October 2011, It was revealed that Jansher was suffering from Parkinsonism and is currently being treated in Pakistan.