His first novel, Every Dead Thing, was met with critical acclaim; it was nominated for the Bram Stoker Award for Best First Novel and went on to win the 2000 Shamus Award for Best First Private Eye Novel (he is the first author outside of the US to have won the award). Connolly's debut introduced readers to the anti-hero Charlie Parker, a former police officer hunting the killer of his wife and daughter. Connolly has since written a further 5 books in the popular Parker series and a non-Parker thriller, as well as venturing outside of the crime genre with the publication of first, an anthology of ghost stories and later, a novel about a young boy's coming-of-age journey through a fantasy realm during World War II England. Several film adaptations of his works are currently in development; the earliest to appear to audiences will be partially based on the short story "The New Daughter", and stars Kevin Costner and Ivana Baquero, previously seen in the acclaimed film Pan's Labyrinth. Connolly is also known to tour extensively to promote the launch of his books. In 2007, Connolly made book-store appearances in Ireland, United Kingdom, United States, Australia, New Zealand, Hong Kong and Taiwan to promote The Unquiet.

Before becoming a full-time novelist, Connolly had worked as a journalist, a barman, a local government official, a waiter and a gofer at Harrods department store in London. After graduating with a B.A. in English from Trinity College, Dublin and a M.A. in Journalism from Dublin City University, he spent five years working as a freelance journalist for The Irish Times newspaper. He quickly became frustrated with the profession, and began to write Every Dead Thing in his spare time. Connolly continues to contribute articles to the paper, most notable of which have been a series of interviews with other established authors.

Although Connolly's novels fall under the crime genre, his stories have become increasingly tinged with supernatural overtones. In the Parker book, The Black Angel, Parker is haunted by the ghosts of his murdered wife and daughter (whether real or imagined), while his search for a missing New York prostitute is linked to the myth of the fallen angels. The setting of Maine in each of his novels, coupled with the combination of suspense and the paranormal, have invited comparisons to the works of Stephen King, although the two authors employ very different styles. Connolly was drawn to the tradition of American crime fiction, because it seemed the best medium through which he could explore the issues of compassion, morality, reparation and salvation. He credits veteran authors Ross Macdonald, James Lee Burke and Ed McBain as major influences, and is often praised for writing in a rich and introspective style of prose rarely exhibited by other authors within the genre.

He is an avid reader and music collector, and also cites cooking and going to the gym as pastimes.

The eighth book in the Charlie Parker series, The Reapers, was published in 2008. It differs from the earlier books in that the story is told from the point of view of Parker's close friends and allies in combat, Louis and Angel. Louis and Angel are an unlikely couple whose quibbles and good humour are sometimes the source of comic relief. Louis is an enigimatic, large black man who was a hired killer but who now seems to be in semiretirement; Angel is a small white man and ex-burglar. They appear episodically throughout the Charlie Parker books as his only close friends, revealing themselves when Parker is in need of help and professional protection from his enemies.

The tenth Parker novel, titled The Whisperers, was published in 2010. Connolly is currently working on a script for the film adaptation of his short story "The Erlking", as well as a sequel to "The Gates".

He began contributing to a weekly blog in January 2006, which can be read on his official website, his MySpace page and Amazon.com's AmazonConnect author feature.

Charlie Parker series

• Every Dead Thing, 1999
• Dark Hollow, 2000
• The Killing Kind, 2001
• The White Road, 2002
• "The Reflecting Eye", a novella included in Nocturnes, his collection of short stories, 2004
• The Black Angel, 2005
• US editions were accompanied by the first of two soundtracks to his novels, featuring tracks by Lullaby for the Working Class, Red House Painters, Hem, Lambchop, Kate Bush, The Go-Betweens, The Walkabouts, Beachwood Sparks, Neko Case, Thee More Shallows, Pinetop Seven, The Triffids, Radar Bros. and The Blue Nile.
• The Unquiet, 2007
• This time, UK editions were accompanied by a second music compilation, featuring tracks by Nickel Creek, Midlake, Low, Willard Grant Conspiracy, The Czars, Efterklang, Hood, Woven Hand, Starless & Bible Black, The National, The Delgados, Jim White, Sufjan Stevens, Espers, Phelan Sheppard.
• The Reapers, May 2008
• The Lovers, 2009
• The Whisperers, 2010

Other novels

• Bad Men, 2003 (Connolly's first stand-alone crime novel)
• The Book of Lost Things, 2006 (Connolly's first non-mystery novel)
• The Gates, 2009

Short story collections

• Nocturnes, 2004 - a collection of supernatural tales book-ended by two novellas, 9 of which are transcripts of stories written for presentation on BBC Radio Four.

Short stories

• "The Inkpot Monkey", 2004 in Like A Charm: A Novel In Voices - an anthology of short stories from 15 mystery writers, (also featured in Nocturnes). Edited by Karin Slaughter.
• "Mr. Gray's Folly", 2005 in Dangerous Women - an anthology of short stories from 17 crime writers. Edited by Otto Penzler.
• "The Cycle", 2005 under the pseudonym Laura Froom (after the titular vampire in a story from Nocturnes) in Moments: Short Stories by Irish Women Writers in Aid of the Victims of the Tsunami. Edited by Ciara Considine.
• "A Haunting", June 2008 in Downturn Tales: Stay-Up-All-Night Stories from Your Favorite Bestselling Authors.