Johnston was educated at Trent University and worked there as an administrator and counsellor. He then became a social worker specialising in the elderly and was an organizer for Ontario New Democratic Party leader Stephen Lewis in the 1970s. He also worked as executive director of community care in Durham Region.
When Lewis left politics, Johnston ran to succeed him as the NDP Member of Provincial Parliament (MPP) for Scarborough West, winning election to the Legislative Assembly of Ontario in a 1979 by-election. Johnston was re-elected in the elections of 1981, 1985 and 1987. The only time he faced a serious challenge was in 1981, when he defeated Progressive Conservative John Adams by 375 votes.
Johnston ran in the 1982 NDP leadership convention, coming in second to Bob Rae. Johnston's leadership bid received support from many on the left-wing of the party. In the legislature, Johnston served as chair of the NDP caucus and chair of the social development committee during the 1985 to 1987 minority legislature in which the NDP held the balance of power. Johnston, unlike Rae, had originally supported a coalition government with the Liberal Party following the election of 1985.
He crusaded for improved benefits to the poor and disabled by attempting to live for a month on a "welfare diet" in 1982, limiting his food budget to that of the average person on welfare. In 1987, he presented a report to the legislature, Toward a New Ontario, which recommended an overhaul of the existing social assistance system.
Johnston was planning to run for the NDP leadership again in 1989, when Rae considered running for the leadership of the federal New Democratic Party. These plans came to nothing, after Rae decided to remain in provincial politics.
After suffering a heart attack, Johnston decided to retire from politics and did not run for re-election in the 1990 election. As that election was won by the NDP, he narrowly missed the opportunity to serve in government as he would have been certain to have received a senior cabinet position had he remained in politics.
Following his departure from politics, Johnston returned to academia serving as chair of the Ontario Council of Regents for the Ontario Colleges of Applied Arts and Technology from 1991 to 1995. He then served as president of the First Nations Technical Institute on the Tyendinaga Mohawk Territory from 1995 until 1998 as well as a member of Trent University's board of directors. Johnston served as president of Centennial College in Scarborough from 1998 until 2004, when he retired to concentrate on the vineyard and winery that he and his wife operate in Prince Edward County.