Magnum P.I. (2018 TV series, season 3)

• Jay Hernandez as Thomas Magnum, a former Navy SEAL who is a security consultant for the successful novelist Robin Masters, living in the guest house on his estate, while also working as a private investigator
• Perdita Weeks as Juliet Higgins, a former MI6 agent who is majordomo to Robin Masters; she and Magnum bicker but become allies
• Zachary Knighton as Orville "Rick" Wright, a Marine veteran and former door gunner, who runs his own tiki bar and is also a playboy
• Stephen Hill as Theodore "T.C." Calvin, a Marine veteran and helicopter pilot who runs helicopter tours of Hawaii and is a member of Magnum's team
• Amy Hill as Teuila "Kumu" Tuileta, the cultural curator of Robin Masters' estate
• Tim Kang as Honolulu Police Department (HPD) Detective Gordon Katsumoto, who dislikes Magnum but usually comes to the team's aid when needed

• Jay Ali as Dr. Ethan Shah
• Christopher Thornton as Kenny "Shammy" Shamberg
• Lance Lim as Dennis Katsumoto

• Kimee Balmilero as Dr. Noelani Cunha
• Shawn Mokuahi Garnett as Flippa
• Dennis Chun as HPD Sergeant Duke Lukela

Following the cancellation of Hawaii Five-0 in early 2020, numerous actors from the series still made appearances throughout the season, most notably Kimee Balmilero who recurred in the season as Dr. Noelani Cunha.[11] In addition Shawn Mokuahi Garnett appeared in two episodes of the season as Flippa[12][13] and Dennis Chun appeared as Honolulu Police Department Sargent Duke Lukela in the seasons fourth episode.[14] When asked about a possible crossover between Magnum P.I. and other Lenkov-verse series MacGyver the writers noted the difficulty in filming one because Magnum P.I. films in Hawaii while MacGyver films in Atlanta, Georgia but left the possibility open.[15]

Co-showrunner, co-developer, and executive producer Peter M. Lenkov was fired by CBS after allegations over a toxic work environment.

On May 6, 2020, CBS renewed Magnum P.I. for a third season, along with eighteen other series including MacGyver, the second and only other remaining series from the Lenkov-verse after the cancellation of Hawaii Five-0.[16] On July 7, 2020, it was revealed that co-showrunner, executive producer, and co-developer Peter M. Lenkov would not have any involvement in the season after being fired from CBS over toxic work environment allegations.[17] Lenkov was originally expected to continue work on the series for another year after signing a three-year deal with CBS Television Studios in 2018.[18][19] Lucas Till who portrays the titular character of Angus MacGyver on MacGyver stated that Lenkov made him suicidal and constantly body shamed him.[20] Lenkov's lawyers initially denied all allegations.[21] Lenkov later responded to the situation by stating "It's difficult to hear that the working environment I ran was not the working environment my colleagues deserved, and for that, I am deeply sorry. I accept responsibility for what I am hearing and am committed to doing the work that is required to do better and be better."[22] Lenkov still received writing credits for various episodes throughout the season written prior to his termination.[23] Numerous production staff moved to the series also following the cancellation of Hawaii Five-0 including co-showrunner of its final season, David Wolkove, a line producer, and editors.[24] Eight different writers wrote episodes throughout the season.[25] Gene Hong replaced Lenkov as co-showrunner joining other co-showrunner Eric Guggenheim who also co-developed and executive-produces for the series.[26] On October 27, 2020, it was reported that the season would have a reduced episode order of sixteen-episodes as a result of the COVID-19 pandemic.[27] The seasons main storylines revolved around a post-COVID 19 pandemic world; however, the effects of the pandemic continue to play a part in storylines throughout the season.[28]

On July 1, 2020, it was revealed that the series was eyeing a delayed mid-August start date to begin filming as a result of the COVID-19 pandemic.[29] Previous seasons had begun filming in July.[30][31] In August 2020, it was announced that the series had been given permission to begin filming but had yet to set a start date;[32] September 14, was later stated as a tentative start date to filming.[33] Filming for the season began two days later on September 16, 2020, with a traditional Hawaiian blessing.[34] COVID-19 protocols were required on set including the wearing of masks, social distancing, COVID testing, and reduced cast and crew on set.[35] Due to the limited numbers of extras allowed on the set, crowd replication visual effects were used as a replacement.[36] Kurt Jones, a cinematographer for the season, who also worked on Hawaii Five-0, directed the fifteenth produced episode of the season.[10]

On October 16, 2020, it was announced that Jay Ali had been cast in a recurring role for the season as Dr. Ethan Shah, a love interest for the character of Juliet Higgins.[37][38] It was later revealed that Lance Lim would also recur throughout the season as Dennis Katsumoto, the son of Detective Gordon Katsumoto.[39] Roger E. Mosley, who portrayed Theodore Calvin on the original Magnum, P.I. reprised his role as John Booky from the first season of the rebooted series.[40] Dennis Chun, who portrayed various minor characters on the original series also returned as Honolulu Police Department Sergeant Duke Lukela.[41] Christopher Thornton, Kimee Balmilero, and Shawn Mokuahi Garnett continue to recur in the series after being introduced in the first season.[42][43] Bobby Lee and Janel Parrish also reprised their roles after both being introduced as characters in the second season.[44][45]