Collins was born and raised in Belfast, Northern Ireland, where he grew up during The Troubles. He was educated at the Royal Belfast Academical Institution before attending the Queen's University of Belfast, where he gained a degree in economics.
After graduating from university, Collins was accepted into the Royal Military Academy Sandhurst, from where he was commissioned into the Royal Signals as a second lieutenant on a short service commission on 2 October 1981. He was promoted to lieutenant with seniority from 7 April 1981. He transferred to the Royal Irish Rangers on 18 October 1982. He switched to a full commission on 22 October 1984, and was promoted captain on 7 October 1985.
He was promoted major on 30 September 1992, and lieutenant-colonel on 30 June 1999. Collins was appointed commanding officer of the 1st Battalion, Royal Irish Regiment in 2001. For a tour of duty in Northern Ireland between October 2001 and March 2002, he was awarded the Queen's Commendation for Valuable Service on 29 October 2002. It was in the capacity of 1 R Irish's commanding officer that he rose to prominence while serving in Iraq.
On 31 October 2003 he was appointed Officer of the Order of the British Empire, for his service in Iraq and was invested on 7 April 2004. Collins was promoted to colonel and moved to the General Staff on 30 June 2003.
As Lieutenant Colonel (Commanding Officer) of the 1st Battalion, Royal Irish Regiment of the British Army, Collins gave a rousing eve-of-battle speech to his troops in Kuwait on Wednesday 19 March 2003. The speech was extemporised, and was recorded in shorthand by a single journalist, Sarah Oliver. No recording or film of the speech exists, Collins told the BBC.
The "Mark of Cain" line from the speech inspired the title of the 2007 Film4 Productions drama The Mark of Cain. In the film a commanding officer makes a speech based on Collins' to his men.
The last episode of the 2008 television series 10 Days to War features a version of the speech performed by Kenneth Branagh as Collins.
He set up the Peace Support College in Sarajevo before becoming DACOS Training at HQ Land Command until his retirement.
After serving in the Iraq War he was accused of mistreatment of Iraqi civilians and prisoners of war. He sued the Sunday Mirror newspapers at the High Court in Belfast and won substantial undisclosed libel damages from them.
He officially left the army on 5 August 2004.
Since leaving the Army, Collins' views on the Iraq conflict and other military issues have been widely sought. In 2007, Collins was host of a three-part documentary called Ships That Changed the World for BBC Northern Ireland. In December 2008 – during an interview on the BBC's Today programme, Collins said that, when he left in 2004, the British Army was already undermanned for existing commitments. In February 2011 Tim Collins appeared on the BBC news programme Panorama in a special entitled 'Forgotten Heroes'. In the documentary, Collins meets veterans struggling to cope with civilian life and sleeps rough on the streets of Brighton with another former soldier.
Collins has been approached by both the Conservative Party and the Ulster Unionist Party to run for Parliament, though has not made any commitment to either party. During the 2005 Ulster Unionist leadership election he was cited by a number of prominent Ulster Unionists as an outside figure who would make a good leader, but Collins declined as he felt he had "no experience of politics." Collins is a signatory of the founding statement of principles of the Henry Jackson Society, which advocates a pro-active approach to the spread of liberal democracy through the world. He has recently been critical of the Iraq war: "the UK and US pour blood and treasure into overseas campaigns which seem to have no ending and no goal ... Clearly I was naive".
In December 2011, it was revealed that Collins was approached to stand as an elected police commissioner for the Conservatives in Kent and originally was standing, however he later dropped out of the race. In August 2014, Collins was one of 200 public figures who were signatories to a letter to The Guardian opposing Scottish independence in the run-up to September's referendum on that issue.
Collins is the Chairman of specialist security company, New Century Consulting.Collins, Tim (2006). Rules of Engagement. London: Review. ISBN 978-0-7553-1375-4. OCLC 62796448.