Lill joined the RNZAF as a 13–course BES cadet entrant in 1959. It was in the RNZAF that he first expressed an interest in amateur dramatics and a fondness for mimicking characters of the BBC's The Goon Show. Throughout his air force career, he involved himself in operatic and theatric productions. He left New Zealand for Britain in 1967.
Some of his many film and television roles include Fall of Eagles (1974), Edward the Seventh (1975), Survivors (1975–77), The Eagle Has Landed (1976), Bad Blood (1981), The Scarlet Pimpernel (1982), as William Knox D'Arcy, the Australian oil pioneer in Persia, in Reilly: Ace of Spies (1983), Rumpole of the Bailey (1983–92), Mapp and Lucia (1985–86), Only Fools and Horses (1989–92), Batman, Red Dwarf (1 episode, 1990s) (1989), Bernard and the Genie (1991), Richard III (1995), Evita (1996), The Opium War (1997), Rebecca (1997) Heartbeat (2002) and The Royal (2003-2011).
Lill twice appeared in the BBC's science fiction series Doctor Who: as Dr Fendelman in the 1977 serial Image of the Fendahl and as Sir George Hutchinson in 1984's The Awakening. After a one-off appearance in the first series of Terry Nation's post-apocalyptic drama Survivors as Charles Vaughan, he became the leading actor for the show's second and third series. Lill portrayed Bertie, the Prince of Wales and later Edward the VII, in the 1978 period serial drama Lillie opposite Francesca Annis as Lillie Langtry. In the 1990s he starred in sitcoms Outside Edge as Dennis and Only Fools and Horses as Cassandra's father, Alan Parry.
Lill played Inspector Bradstreet in the 1986 Sherlock Holmes adaptation of The Man with the Twisted Lip. He also appeared as the drunken MP Sir Talbot Buxomly who died onscreen in an episode of Blackadder the Third and as the Beadle in Blackadder's Christmas Carol. He also appeared as Sir Reuben Astwell in "The Underdog" (1993) of the detective series Poirot. He appeared in Red Dwarf, playing a rogue simulant in "Gunmen of the Apocalypse", in Bertie and Elizabeth as Clement Attlee (2002) and from 2003 until 2011 Lill starred in the ITV1 drama The Royal as Consultant General Surgeon Mr. Rose. Lill has also worked in radio theatre with one of his memorable characters being the brother in Ron Blair's play The Christian Brothers. In 2014, he appeared in 24: Live Another Day.
In February 2016, Lill appeared in series 18 episode 4 " A Dying Art" of the ITV crime drama Midsomer Murders.
Lill appeared in Series 2 Episode 5 of "Shakespeare and Hathaway: Private Investigators" in March 2019.
|1976||The Eagle Has Landed||Churchill's aide|
|1981||Bad Blood||Ted Best|
|1982||The Scarlet Pimpernel||William Knox D'Arcy|
|1988||Salome's Last Dance||Tigellenus / Chilvers|
|1989||Batman||Bob the Cartoonist|
|1993||Red Hot||The Rector|
|1995||Richard III||Lord Mayor|
|1996||Element of Doubt||Simon|
|1997||Fierce Creatures||Woman's Husband|
|1997||Mrs Dalloway||Doctor Holmes|
|1998||What Rats Won't Do||Moot Master|
- "From Airman to Actor". RAF News. RnR page 8 (1399). 15 July 2016. ISSN 0035-8614.
- "Only Fools and Horses Uncovered". BBC. BBC. 24 September 2014. Retrieved 21 September 2016.
- Reinhart, Mark S (2012). "8: Barman 1989". The Batman Filmography (2 ed.). North Carolina: McFarland & Co. p. 99. ISBN 978-0-7864-6891-1.
- "Denis Lill". BFI Film Forever. Retrieved 21 September 2016.
- Sowden, Steve (1 June 2011). "Well-known actor at church fete at Montacute House". This is the West Country. Retrieved 21 September 2016.
- "In the Hot Seat Denis Lill". Somerset Live. 21 January 2012. Retrieved 21 September 2016.