McCann was born and has lived most of his life in Derry. He was educated at St. Columb's College in the city. McCann is prominently featured in the documentary film The Boys of St. Columb’s.

He was involved with the Irish Workers Group, a Trotskyist organisation, for a time in the 1960s .

As a young man he was one of the original organisers of the Northern Ireland Civil Rights Association (NICRA). His political contemporaries included Bernadette Devlin, for whom he served as an election agent. McCann has always been critical of other politicians in Derry, and through-out Ireland North and South whose politics were based on cultural-identity rather than on class analysis. He personally witnessed and participated in many of the key events of the early part of the Troubles, including the Battle of the Bogside in August 1969 and Bloody Sunday in January 1972.

Later he worked as a journalist for the Sunday World newspaper and contributed to the original In Dublin magazine among others. He currently writes for the Belfast Telegraph and the Derry Journal, and has for many years written a column for the Dublin-based Hot Press magazine. He is a frequent commentator on the BBC, RTE and other media.

A Trotskyist and outspoken atheist, he is a prominent member of the Socialist Workers Party in Ireland, and in recent Northern Ireland elections has stood as a candidate for the Socialist Environmental Alliance. Previously he stood unsuccessfully as a Labour Party candidate in the 70s.He is also Chair of his local branch of the NUJ, and Vice-Chair of Derry Trades Council.

As a political activist, McCann has lent his support and considerable oratorical skills to many causes, including campaigns in support of abortion rights, immigrants and gay marriage. Much of his journalistic work reflects what he himself describes as a "shuddering fascination" with religion which, when coupled with his profound scepticism, has made it a topic to which he has often returned.

McCann was the partner of Mary Holland, the late journalist for The Observer, and The Irish Times. He has a daughter from that relationship, Kitty, who is now a journalist for The Irish Times, and a son, Luke, who is also a journalist working for Hola magazine in Spain. Fellow SWP member and academic Goretti Horgan has been his partner since the mid 1980s and they have an adult daughter, Matty.

In the 2002 film Bloody Sunday, McCann's character is played by the Irish actor Gerard Crossan.

In March 2008, McCann spoke with National Public Radio in the U.S. about the solidarity between the Catholic civil rights movement in Northern Ireland and the African American Civil Rights movement.

McCann was tried in Belfast in May/June 2008 for his actions as one of the Raytheon 9. The jury unanimously acquitted McCann, and all the other defendants, of charges of "Criminal Damage" on property belonging to multinational arms company Raytheon. The judge dismissed charges of "Affray" after hearing the prosecution evidence. However, McCann was found guilty of the theft of 2 computer discs. For this he received a 12 month conditional discharge.

In a statement outside the court McCann said:

"[we] have been vindicated. ... The jury have accepted that we were reasonable in our belief that the Israeli Defence Forces were guilty of war crimes in Lebanon in the summer of 2006. The action we took was intended to have, and did have, the effect of hampering or delaying the commission of war crimes."

McCann now writes a column for the Sunday edition of the Derry Journal.

McCann is also a supporter of Derry City FC

His books include:

• War And An Irish Town (1973), an account of his upbringing in Derry and the beginnings of the civil rights movement in the region
• War and Peace in Northern Ireland - a critique of the political situation in Northern Ireland, notable for its criticism of both nationalism and unionism
• Dear God - The Price Of Religion In Ireland, a radical debunking of Christianity

He has also edited two books on Bloody Sunday:

• Bloody Sunday: What Really Happened (1992)
• The Bloody Sunday Inquiry: The Families Speak Out (2005).