McInnerny was born in Cheadle Hulme, Stockport, the son of Mary Joan (née Gibbings) and William Ronald McInnerny. He was brought up in Cheadle Hulme, Stockport in Cheshire, and Stroud, Gloucestershire, and educated at Marling School, Stroud, and Wadham College, Oxford.

McInnerny has worked as a dramatic actor, appearing on stage in the original production of Pravda with Anthony Hopkins, and on television in Edge of Darkness (1985) where he played an anarchist.

He was John Clay in the Granada The Adventures of Sherlock Holmes episode entitled "The Red-Headed League". He has also starred in Wetherby (1985), Erik the Viking (1989), a film production of Shakespeare's Richard III (1995), FairyTale: A True Story (1997), Notting Hill (1999), 102 Dalmatians (2000), The Emperor's New Clothes (2001), and Severance (2006).

He also played Dr. Frank-N-Furter in the 1990 West End production of The Rocky Horror Show, alongside Adrian Edmondson as Brad and Ed Tudor-Pole as Riff-Raff. His performance can be heard on the soundtrack album of this production. In 1989, he co-starred with Kate Bush in the music video for her song "This Woman's Work".

He also appeared in the Westlife video for "Uptown Girl", along with Robert Bathurst, Crispin Bonham-Carter, Ioan Gruffudd and James Wilby. In 1999, he played Tony Hawes, a senior employee with Barings Bank alongside Ewan McGregor in the film Rogue Trader.

In 2004, he joined the cast of the BBC/Kudos spy drama Spooks for its third season, playing Oliver Mace, a semi-regular character.

In 2006, he starred in the BBC adaptation of The Line of Beauty as Gerald Fedden. That same year, he also starred in One Day, a short film shown at various international film festivals.

In summer 2007 he played Iago in Othello at Shakespeare's Globe on Bankside in London.

In April 2008, he guest starred in the episode "Planet of the Ood" of the fourth series of Doctor Who.