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 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.

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.

[14] After Paul Robinson (Stefan Dennis) was pushed from the Lassiter's Hotel mezzanine, Mark was introduced as the lead detective on the case.

At one point Kate's having driving lessons, and I'm working as a detective who's moved up through the ranks quickly, so we do seem quite different in age, but it's better left unsaid I think!

[18] McGregor told Inside Soap that Mark did not stop loving Kate and that since he had time to think about it, the issue was not as "black and white" as he first thought.

[18] In April 2011, a TV Week columnist reported that Mark would have a one-night stand with Kate's best friend, Jade Mitchell (Gemma Pranita).

[20] At the same time, Mark also became involved in a police corruption storyline, after he reported Constable Wes Holland (Daniel Hamill), for taking bribes.

[21] McGregor told an Inside Soap writer, "Mark's life has been turned upside down since he blew the whistle on Constable Holland", adding that he was now being treated like he did the wrong thing.

[31][32][33] TV Week's Erin Miller stated, "Fans of Scott's character, Detective Mark Brennan, will be overjoyed to discover he didn't die in witness protection as they'd been led to believe – and his return could potentially see him reunite with ex-girlfriend Kate".

[48] On 2 August 2019, Alice Coster of the Herald Sun reported that Mcgregor would be leaving Neighbours, following news that he and his wife were expecting their second child.

[49] Herbison stated, "Mark Brennan has been the anchor of so many pivotal stories on Neighbours over the years from Erinsborough's super cop to serial heartbroken romantic.

[52] Mark decides to leave Erinsborough to help Tyler take care of their mother in Adelaide, after her Huntington's disease worsens.

[53] The character returned on 17 February 2020, having been temporarily transferred back to Erinsborough to deal with a gun ring that Ned Willis (Ben Hall) has got caught up in.

Mark eventually catches the ringleader of the illegal races, Garland Cole (Jack Finsterer), after he asks Lucas to rebirth some cars for him.

Paige Smith moves in with Mark, and he helps her when she is revealed to be the secret daughter of Lauren Turner (Kate Kendall) and Brad Willis (Kip Gamblin).

Brennan asks Lauren if he can expose Matt's corruption to put Dimato in prison, which causes her son, Bailey (Calen Mackenzie), to lash out at him.

Mark learns from Steph's former girlfriend Belinda Bell (Nikki Shiels) that one of her patients Ari Philcox (Dylan Watson) is behind the release of the snakes.

Paige's son Gabriel Smith (Kian Bafekrpour) is kidnapped by Louise and she tells Mark to bring her money and a passport for his safe return.

Mark and Aaron's sister Chloe Brennan (April Rose Pengilly) arrives for an extended visit and hopes to reconnect with her brothers.

Broken by the thought of losing someone else, Mark pushes Elly away, considers calling off the wedding and begins visiting Kate's grave.

Leo's business partner Roxy Willis (Zima Anderson) is frustrated by lack of progress in investigation and berates Mark when he comes to the bar to celebrate his birthday.

Despite their concerns about living in different states, their romance progresses fast and Paige proposes to Mark so they can get married as part of the Lassiter's wedding expo.

To coincide with the character's return in February 2014, the serial released a series of webisodes called Brennan on the Run that focus on Mark's time in witness protection.

[56] Producers announced that viewers would be able to choose the outcome of the story and take control of Mark "as he faces challenges, dodges danger and encounters a little bit of romance".

[58] The webisodes were shot using a Red Scarlet camera by digital producer Ric Forster, which gave them "a different, edgier look and feel" compared to the show's usual episodes.

[62] In April, the magazine's letters editor, Sarah Ellis, said she would like to buy a calendar featuring Mark, as she would love to have the "hunky policeman [...] adorning the walls of Inside Soap towers.

"[67] Nicky Branagh from Studentbeans.com included Mark in her list of the "Top ten hottest Aussie soap guys" and she stated "As Ramsay Street's resident hottie back in 2010, Scott McGregor graced Neighbours as the coolest kind of police investigator (he rode a motorbike).

"[71] In 2016, Crick (All About Soap) criticised the character for arresting his fiancée on their wedding day, saying "We all know Mark Brennan can be a bit strait-laced when it comes to being a copper (has there ever been a man in Ramsay Street who takes the law so seriously?

"[74] McMullan praised McGregor's acting performances during the darker stories and hoped the writers would let Mark be happy in the future by giving him a child.