The Caucus was formed several years after the demise of The Waffle, an earlier left-wing faction which had been forced to leave the NDP in the mid-1970s.
It was formed by 150 to 200 members in the lead up to the NDP's 1979 federal convention to which it proposed a series of radical resolutions promoting greater public ownership of the economy and other socialist and left-wing measures. John Rodriguez, Member of Parliament for Nickel Belt, was the leading spokesperson for the caucus. Another notable founding member of the Left Caucus was Ontario NDP MPP and future provincial finance minister Floyd Laughren.
Rodriguez said the caucus different from the Waffle in that its ultra-left elements were a minor part and as it was willing to abide by the final resolutions of the convention, whether it agrees with them or not. However, Rodriguez warned that the Left Caucus would be outspoken if resolutions it favours passed the convention but were ignored by the party's leadership.
The only one of the caucus' resolutions to be approved by the 1979 convention, however, was one protesting the jail sentence handed to Jean-Claude Parrot, president of the Canadian Union of Postal Workers for leading a wildcat strike.
Prior to the 1983 federal convention, the Left Caucus again pushed for the party to adopt an economic policy focussed on nationalization of industry and issued a document decrying "the half-measures of a mixed economy" and calling for "a socialist industrial strategy based on public sector ownership of the decisive sectors of the economy." The caucus unsuccessfully opposed the adoption of a new "statement of principles" by the NDP in 1983 which watered down the party's commitment to socialism and the planned economy.
In the mid-1980s the Caucus was involved with the Campaign for an Activist Party which attempted unsuccessfully to elect a slate led by Judy Rebick to the executive of the Ontario NDP.
Simon Rosenblum, later a vice-president of the ONDP and chief of staff to provincial finance minister Floyd Laughren, was a leader of the caucus in the early 1980s and authored a resolution that called on an NDP government to utilise "the central role of public ownership in the development of our primary resource industries". By the end of the 1980s the Left Caucus was led by members of the "Forward" Group such as Gord Doctorow, Wayne Roberts, Harry Kopyto and George Ehring.
The Caucus declined from 1990 to 1995 while the Ontario NDP was in government under Bob Rae with publication of the Left Caucus Newsletter ceasing in 1994. In 1993, leading Left Caucus members Harry Kopyto, Gord Doctorow and Lois Bedard organised an open letter signed by 11 party members including three dissident MPPs criticising the Rae government for being too close to corporate interests and alienating itself from labour.
- "Left Caucus fills Waffle shoes in battling NDP" by Kirk Makin, Globe and Mail, November 24, 1979
- "No more party of the underdog By abandoning the unions and most of its traditions, Bob Rae is staking out a radical new vision for the NDP. For better or worse, this achievement will be his greatest legacy", by Thomas Walkom, Toronto Star, May 22, 1993
- "NDP convention finds radicals in left field" by Jeffrey Simpson, Globe and Mail, November 26, 1979
- "Lack of a seat for Rae helps MPP's chances for Cassidy job" by Ian Mulgrew, Globe and Mail, July 6, 1981
- THE NEW PARTY AFTER 50 YEARS, WHAT'S AHEAD FOR THE NDP?" by Judy Steed, Globe and Mail, June 25, 1983.
- "What art thou NDP?", Toronto Star, June 20, 2011
- "NDP poised to zoom into big time" By Martin Cohn, Toronto Star, July 27, 1987
- "HAS THE LEFT SOLD OUT? The social-democratic movement is in massive disarray, its members disgruntled and splintered. Thousands are casting about for a new political home, though many see no alternative on the horizon" by Geoffrey York, Globe and Mail, July 17, 1993
- "Deadly sins of the NDP", By Bill Walker Toronto Star, September 18, 1993
- (ONDP) LEFT CAUCUS NEWSLETTER—1985-1994
- "Anti-Rae letter forces others to back Premier Presidents of CAW, NDP distance selves from Kopyto-authored attack" by Richard Mackie, Globe and Mail, March 1, 1993