James-Collier was born in Salford, Greater Manchester, as Robert Collier, but changed his name to Robert James-Collier to comply with the rules of Equity. He was educated at St Patrick's Roman Catholic High School in Eccles, Greater Manchester.
James-Collier has two degrees; he studied business at the University of Huddersfield and marketing at the University of Manchester Institute of Science and Technology.
Having no experience as an actor, James-Collier auditioned for the series Down to Earth. Stars Ricky Tomlinson and Denise Welch were convinced by his talent, and he got the part of the womanising pub landlord. In 2006 he appeared in New Street Law and played a small part as Stud in Shameless series 3, episode 5 under the name of Rob Collier.
In 2006 he joined Coronation Street as one of the brothers buying a share of the local underwear factory. He left the show in October 2008. In an interview in early 2008, James-Collier announced he was leaving the soap but was planning to come back in the future. However, in June 2008, it was reported that the character of Liam was set to be 'brutally' killed off in the soap as part of a murder-mystery plotline where three well known residents would fall under suspicion for the crime. One of these would be Tony Gordon played by Gray O'Brien, who was in a relationship with Carla Connor played by Alison King in the show.
After leaving Coronation Street, James-Collier had a leading role as under butler (formerly first footman) Thomas Barrow in Downton Abbey, from 2010 to 2015, and has appeared in the West End stage version of Calendar Girls.
James-Collier played Kevin O'Dowd in ITV's 2016 crime drama, The Level. In 2017, James-Collier starred in British horror film, The Ritual, in which The Guardian described his performance as "a fierce screen presence". He will also play a small role in Jessica Hynes directorial debut, The Fight. 
James-Collier modelled for Argos, where he appeared in the Autumn/Winter 2007 and Spring/Summer 2008 catalogues.
He won the Sexiest Male award at the 2007 and 2008 British Soap Awards in addition to "Best Exit" at the 2009 British Soap Awards. He won Sexiest Male and Best Newcomer at the 2007 Inside Soap Awards.
|2005||Perfect Day||Alex||TV film|
|2005||Down to Earth||Nicky Christy||TV series (Series 5)|
|2006||Shameless||Stud/Jim||TV series (1 episode: "Episode No. 3.5")|
|2006||Casualty||Martin Stonewall||TV series (1 episode: "Secrets and Lies")|
|2006||Dalziel and Pascoe||Simon Thewlis||TV series (1 episode: "The Cave Woman: Part 2")|
|2006-08||Coronation Street||Liam Connor||TV series (330 episodes: 2006–2008)
Won – Best Exit – British Soap Awards
Won – Sexiest Male (2008) – Inside Soap Awards
Won – Sexiest Male (2007) – Inside Soap Awards
|2010-15||Downton Abbey||Thomas Barrow||TV series
Won – Screen Actors Guild Award for Outstanding Performance by an Ensemble in a Drama Series (2012 -2014)
|2011||Moving On||Clive||TV series (1 episode: "The Milkman")|
|2012||Love Life||Joe||TV series|
|2012||Spike Island||Mr Milligan||Film|
|2013||Moving On||Aiden Evans||TV series (1 episode: "Visiting Order")|
|2015||A Christmas Star||Pat McKerrod||Film|
|2016||The Level||Kevin O'Dowd||TV series|
|2016||The Attendant||Alex||Short film|