The veteran socialist has led from the front in the civil rights and trade union movements, particularly in his home city of Londonderry.
His political activities even cost him his place as a psychology student at Queen's University in Belfast, when he was expelled in 1965.
His voice is an unmistakable one, be it on the airwaves or in newspaper print.
And he is no stranger to the ballot box, having contested his first election in 1969.
He stood in Foyle for the Northern Ireland Labour Party that year in the Stormont general election.
But he lost out to the independent nationalist John Hume, who went on to lead the Social Democratic and Labour Party (SDLP).
A long-time opponent of military action right back to the days of the Vietnam War, he landed himself in court over a 2006 protest at the Derry operation of the US defence firm Raytheon.
He was acquitted of a charge of destroying property belonging to the missile manufacturer, but he was convicted of stealing discs from the firm.
Mr McCann's climb to Stormont has been a slow one.
He started building his power base in Foyle in the 2003 Northern Ireland Assembly election, standing for the Socialist Environmental Alliance, and was on the ballot paper in 2007, 2011 and 2016.
In between times, he fought Westminster polls in 2005 and 2010.
Mr McCann's electoral breakthrough finally came with People Before Profit (PBP), 47 years on from his first poll.
While the party does not have a leader as such, he assumes the role of its front man during assembly election campaigns.
In 2016, he took a seat from the SDLP in Foyle.
Mr McCann was one of two PBP members elected to Stormont for the first time last May, with Gerry Carroll topping the poll in West Belfast at the expense of Sinn Féin.
The anti-austerity PBP held seats in the Dáil in the Republic of Ireland, but that was the first time it had gained a parliamentary presence north of the border.
After a long time shouting on the outside, Mr McCann's voice was finally heard echoing around the chamber.
Now 73, he is back on the streets of Derry with his loudhailer, trying to keep hold of his seat.
For decades, Eamonn McCann has been at the heart of the politics of protest in Northern Ireland.