Born and raised in the New York City borough of Manhattan, McDonagh received her Bachelor of Arts from Hunter College and her Master of Fine Arts from Columbia University, where she co-founded and edited the Columbia Film Review. She was simultaneously working in the publicity department of the New York City Ballet under George Balanchine and Peter Martins, eventually becoming head of publicity. McDonagh's Irish-emigrant grandparents owned The Moylan Tavern, comedian and habitué George Carlin's real-life basis for the same-name bar on the 1994-95 Fox Broadcasting sitcom The George Carlin Show.
While writing articles and reviews for numerous publications, including Film Comment, Film Quarterly, Premiere, Entertainment Weekly, and Fangoria, McDonagh published her first book, the auteur study Broken Mirrors/Broken Minds: The Dark Dreams of Dario Argento (1991), which grew out of her master's thesis.
After leaving New York City Ballet to pursue a writing career, McDonagh taught film as an adjunct professor at Hunter College and Brooklyn College, during which time she completed Filmmaking on the Fringe: The Good, The Bad, and the Deviant Directors and The 50 Most Erotic Films of All Time. Her freelance work during this period included film pieces for The New York Times.
She became senior movie editor of the TV Guide website in 1995, while continuing to contribute essays to such anthologies as the British Film Institute's The BFI Companion to Horror (Cassell, 1996), Fantasy Females (Stray Cat Publishing, 2000), Zombie (Stray Cat Publishing, 2000), and The Last Great American Picture Show (Amsterdam University Press, 2004), as well as to numerous film guides. In the mid-2000s, she wrote an occasional column on dance movies for the British magazine Dance Now.
Her book Movie Lust, third in the Sasquatch Books series begun with Book Lust by Nancy Pearl and Music Lust by Nic Harcourt, was published August 28, 2006. Later that year, she became the founding vice-president of the Alliance of Women Film Journalists. She is also a member of the New York Film Critics Online.
McDonagh wrote the TV Guide website's twice-weekly column FlickChick; helped initiate the magazines weekly podcast, TV Guide Talk; and co-starred with fellow editor/critic Ken Fox in a Friday vodcast, Movie Talk. She left TV Guide in October 2008 and subsequently launched the website Miss FlickChick and its accompanying blog.
In 2014 McDonagh created the company 120 Days Books to republish rare 1970s and 1980s gay-erotica genre novels, beginning with a pair of two-in-one volumes: the crime thrillers Man Eater and Night of the Sadist and the supernatural fantasies Vampire's Kiss and Gay Vampire.
McDonagh provides interviews and second-channel commentary on DVD / Blu-ray releases, including for director Paul Schrader's Blue Collar, and liner notes, including for the Criterion Collection releases The Tunnel, The Innocents Kuroneko and the paired Corridors of Blood/The Haunted Strangler, and Arrow Video's Dressed to Kill.
She contributed weekly commentary as the American correspondent for British Armed Forces Radio in 2004.
McDonagh has appeared on panels for the Film Society of Lincoln Center and the Museum of the Moving Image. She has lectured at the Huntington (New York) Arts Center, the Jyväskylä (Finland) Arts Festival, the Morbid Anatomy Museum, and elsewhere, and speaks at horror-film conventions, reflecting one of her specialties.
She also specializes in erotic cinema, appearing as an expert in that capacity in the documentary The 100 Greatest Sexy Moments for the UK's Channel Four.
Other television appearances include NBC's Today and G4's Filter, and such documentaries as Scream and Scream Again: A History of the Slasher Film for the BBC; Night Bites: Women and Their Vampires for WE: Women's Entertainment; Dario Argento: An Eye for Horror for IFC; and the Bravo miniseries 100 Scariest Movie Moments and its 2006 sequel, 30 Even Scarier Movie Moments; 2008's Zombiemania; and, in 2009, Pretty Bloody: The Women of Horror, for Canada's Space network.
McDonagh served on the five-member jury judging films in competition at the 2008 New York Asian Film Festival and on the jury as well for the 2008 New York City Horror Film Festival.
A character in one scene of writer-director Lucky McKee's movie May (2002) can be seen reading McDonagh's Broken Mirrors/Broken Minds, as does the character Domini in the final issue (#18, April 1994) of the Marvel Comics supernatural series Nightstalkers.
In 2016, McDonagh was nominated for a Rondo Hatton Classic Horror Award for Best Commentary for her work on Synapse Films' DVD release of Dario Argento's film Tenebrae.