Mark Brennan (also referred to as just Brennan) is a fictional character from the Australian soap opera Neighbours, played by Scott McGregor. He made his first appearance during the episode broadcast on 30 August 2010. McGregor initially had a four-week guest contract, but this was increased to six months. Shortly after, it was announced that McGregor's contract had been extended again to twelve months and that he would become a regular cast member. However, after his contract negotiations broke down McGregor was written out of the show, departing on 22 June 2011. He reprised his role for a brief return from 20 May 2013, before returning on 5 February 2014. McGregor departed Neighbours for a second time on 15 October 2019, but returned for a guest stint as part of the show's 35th anniversary celebrations on 17 February 2020.

Mark is introduced as the lead detective in the attempt on Paul Robinson's (Stefan Dennis) life. He receives conflicting information from the suspects he interviews, and struggles to identify the culprit. McGregor enjoyed being involved in the storyline, as he had never had that much responsibility before. To prepare for the role, McGregor researched his character's job. Mark is portrayed as a charming perfectionist. He is dedicated to his job in the police force and often puts his work ahead of everything else. As a consequence, he struggles to balance his work and personal lives. McGregor found that he shared some similarities with Mark and he enjoyed getting into character.

Producers soon established a relationship between Mark and Kate Ramsay (Ashleigh Brewer). The couple faced various obstacles, including Kate's jealous ex-boyfriend, an age gap, and Mark's focus on his work. Mark later has a one-night stand with Jade Mitchell (Gemma Pranita), and reports a fellow officer for corruption, which results in his life being threatened. The character left for witness protection during a special real-time episode. Following McGregor's decision to reprise the role, Mark's later storylines saw him reunite with Kate and deal with her death. Producers then paired him with Paige Smith (Olympia Valance) and his parents and siblings were later introduced. The character has been mostly well received by critics and viewers during his time on the show.

On 16 June 2010, reporters from the Herald Sun confirmed that model and television presenter Scott McGregor had begun filming a guest appearance in the serial two days prior. It was later announced that McGregor would be playing Detective Mark Brennan. McGregor was originally given a four-week guest contract, but this was then increased to six months. McGregor said that Mark is his "first decent part" after appearing in bit roles in other Australian dramas, such as Underbelly. McGregor's contract was then increased again to twelve months when Mark proved popular with viewers. McGregor told a Herald Sun reporter that he was enjoying working on Neighbours, but he did not "realise how full-on it is."

"Detective Mark is a crime fighter, making Erinsborough a betterplace to live by taking down the bad guys without so much as breaking a sweat!"

Shortly after he was cast, McGregor said that Mark was "a no-nonsense type. Work is his life, and when he's not at work, he's thinking about work". McGregor told the show's official website that he did some research on police officers to prepare for the role. The writers also went through their research with him. Jackie Brygel from TV Week described Mark as being "handsome" and a "charming investigator." Mark is a perfectionist, who puts his work ahead of everything else and he struggles to hold down a relationship because of it. His dedication to his job means that he does not have much time to socialise. When McGregor was asked if there were any similarities between him and Mark, he said that there was and that he liked getting into character.

Mark struggles with balancing his career and personal life and McGregor told Nate Micó from SX News that he would like to see Mark either grow in his job or get "tempted by the wrong side of the law" in the future. Mark manages to develop friendships with some of the other male characters, like Lucas Fitzgerald (Scott Major). Of this, McGregor said "Even though we're friends there are always references to me being a cop and they're always quite wary." When Mark becomes involved in an illegal street racing investigation, he expresses an interest in buying a motorbike. Inside Soap said they would like to see Mark on the road letting his "wild side" out. McGregor said he "loved" the storyline as he was given the chance to ride a motorbike.

After Paul Robinson (Stefan Dennis) was pushed from the Lassiter's Hotel mezzanine, Mark was introduced as the lead detective on the case. Mark had his "work cut out for him" as he began questioning the suspects, as they each pointed the finger at someone else. He also received conflicting information as everyone had their own suspicions as to who committed the crime. McGregor revealed that Kate Ramsay (Ashleigh Brewer) had given Rebecca Robinson (Jane Hall) a false alibi, Lyn Scully (Janet Andrewartha) points out that Diana Marshall (Jane Badler) earring was found at the crime scene and Andrew Robinson (Jordan Smith) did not help himself with his attitude towards the investigation. Of the case, McGregor said "Brennan is in the thick of the investigation so it's pretty exciting, I've never had so much responsibility." McGregor later explained that it had been "great" to be involved in the storyline, even though he did not share many scenes with Dennis, as he was in a hospital bed. Rebecca was revealed to have been the one who pushed Paul and McGregor said that he knew early on in the storyline that she was the culprit. However, Mark did not know that Rebecca was responsible as he had been thrown by the false alibis. McGregor later said that his first week on Neighbours, filming the Who Pushed P.R? storyline, had stuck in his mind as there were many long interview scenes he had to get through.