| Final | 19:49 | time | | gap | L | | 1. |  Australia | 3:19.38 | WRR | | 3 | 1. | | | Jack Cartwright 48.12s, Kyle Chalmers 46.98s, Madison Wilson 52.25s, Mollie O'Callaghan 52.03s | | 2. |  Canada | 3:20.61 | NR | | 5 | 2. | | | Joshua Liendo 48.02s, Javier Acevedo 47.96s, Kayla Sanchez 52.52s, Penny Oleksiak 52.11s | | 3. |  United States | 3:21.09 | | | 4 | 3. | | | Ryan Held 47.93s, Brooks Curry 47.72s, Torri Huske 52.70s, Claire Curzan 52.84s | | 4. |  Great Britain | 3:22.44 | | | 7 | 4. | | | Tom Dean 48.25s, Lewis Burras 47.86s, Anna Hopkin 53.27s, Freya Anderson 53.06s | | 5. |  Netherlands | 3:24.24 | | | 6 | 5. | | | Stan Pijnenburg 48.80s, Jesse Puts 48.29s, Tessa Giele 54.58s, Marrit Steenbergen 52.57s | | 6. |  Brazil | 3:24.78 | Sa | | 1 | 6. | | | Gabriel Santos 48.73s, Vinicius Assunção 48.03s, Giovanna Diamante 53.98s, Stephanie Balduccini 54.04s | | 7. |  Italy | 3:25.83 | | | 2 | 7. | | | Lorenzo Zazzeri 48.69s, Alessandro Miressi 47.50s, Silvia Di Pietro 54.19s, Chiara Tarantino 55.45s | | 8. |  China | 3:26.92 | | | 8 | 8. | | | Hong Jinquan 48.88s, Wang Changhao 48.86s, Lao Lihui 54.60s, Ai Yanhan 54.58s |
| 122 Competitors from 27 Nations. | | World Record: United States 3:19.40 Gwangju, South Korea 27 July 2019 | | World Championship Record: United States 3:19.40 Gwangju, South Korea 27 July 2019 |
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Heats - Friday, 24th July 09:53 | | Best 8 times advanced to Final. |
| Heat 4 | | time | | gap | L | Rk. | | 1. | United States | 3:43.16 | Q | | 3 | 1. | | | Ryan Murphy 52.40s, Lilly King 1:06.79, Michael Andrew 50.69s, Erika Brown 53.28s | | 2. | Great Britain | 3:43.64 | Q | | 4 | 3. | | | Medi Harris 1:00.09, James Wilby 58.94s, Jacob Peters 51.49s, Anna Hopkin 53.12s | | 3. | Australia | 3:44.58 | Q | | 5 | 4. | | | Isaac Cooper 53.77s, Matthew Wilson 1:00.27, Brianna Throssell 57.51s, Meg Harris 53.03s | | 4. |  Germany | 3:46.54 | Q | | 2 | 8. | | | Ole Braunschweig 53.90s, Anna Elendt 1:05.59, Angelina Köhler 58.06s, Jan Eric Friese 48.99s | | 5. | Brazil | 3:48.07 | | | 7 | 9. | | | Guilherme Basseto 54.69s, João Gomes Júnior 1:00.15, Giovanna Diamante 58.31s, Stephanie Balduccini 54.92s | | 6. |  Israel | 3:48.39 | | | 6 | 10. | | | Anastasia Gorbenko 1:00.08, Kristian Pitshugin 1:01.55, Tomer Frankel 51.71s, Daria Golovaty 55.05s | | 7. |  Latvia | 3:57.77 | Nr | | 0 | 15. | | | Feldbergs 55.65s, Daniils Bobrovs 1:02.69, Ieva Maļuka 1:02.29, Gabriela Ņikitina 57.14s Ģirts | | 8. |  Slovakia | 3:58.67 | | | 8 | 17. | | | Tamara Potocká 1:04.91, Nikoleta Trníková 1:10.14, Ádám Halás 54.02s, Matej Duša 49.60s | | 7. |  Angola | 4:20.77 | | | 9 | 25. | | | Salvador Gordo 1:01.39, Maria Freitas 1:19.67, Lia Lima 1:06.72, Henrique Mascarenhas 52.99s | | -. |  Estonia | DSQ | | | 1 | -. | | | Armin Evert Lelle 55.84s, Eneli Jefimova, Alex Ahtiainen, Aleksa Gold |
| Heat 3 | | time | | gap | L | Rk. | | 1. | Netherlands | 3:43.48 | Q | | 3 | 2. | | | Kira Toussaint 1:00.45, Arno Kamminga 58.70s, Nyls Korstanje 51.12s, Marrit Steenbergen 53.21s | | 2. | China | 3:44.92 | Q | | 4 | 5. | | | Xu Jiayu 53.33s, Yu Jingyao 1:06.73, Wang Changhao 51.23s, Cheng Yujie 53.63s | | 3. |  Japan | 3:45.08 | Q | | 6 | 6. | | | Ryosuke Irie 53.05s, Reona Aoki 1:06.49, Naoki Mizunuma 50.92s, Rika Omoto 54.62s | | 4. | Italy | 3:46.44 | Q | | 5 | 7. | | | Michele Lamberti 54.40s, Arianna Castiglioni 1:05.82, Elena Di Liddo 57.83s, Manuel Frigo 48.39s | | 5. | Canada | 3:48.93 | | | 7 | 11. | | | Javier Acevedo 54.80s, Rachel Nicol 1:07.28, Katerine Savard 58.57s, Yuri Kisil 48.28s | | 6. |  Greece | 3:49.13 | | | 2 | 12. | | | Evangelos Makrygiannis 54.38s, Konstantinos Meretsolias 1:00.35, Anna Ntountounaki 58.88s, Theodora Drakou 55.52s | | 7. |  South Africa | 4:00.65 | | | 8 | 18. | | | Olivia Nel 1:03.35, Brenden Crawford 1:01.33, Clayton Jimmie 56.05s, Stephanie Houtman 59.92s | | 8. |  Bahamas | 4:15.19 | | | 0 | 22. | | | Lamar Taylor 58.58s, Izaak Bastian 1:04.72, Zaylie Thompson 1:10.74, Lillian Higgs 1:01.15 | | -. |  Lithuania | DSQ | | | 1 | -. | | | Erikas Grigaitis 55.27s, Kotryna Teterevkova 1:08.16, Deividas Margevičius, Rūta Meilutytė | | -. |  Peru | DNS | | | 9 | -. | | | |
| Heat 2 | | time | | gap | L | Rk. | | 1. |  Singapore | 3:54.43 | | | 0 | 14. | | | Quah Zheng Wen 55.26s, Nicholas Mahabir 1:00.30, Letitia Sim 1:01.88, Amanda Lim 56.99s | | 2. |  Chinese Taipei | 3:58.13 | Nr | | 2 | 16. | | | Chuang Mu-lun 55.51s, Wang Hsing-hao 1:03.33, Hsu An 1:02.65, Huang Mei-chien 56.64s | | 3. |  Thailand | 4:09.28 | | | 1 | 20. | | | Jinjutha Pholjamjumrus 1:08.22, Dulyawat Kaewsriyong 1:05.69, Navaphat Wongcharoen 54.08s, Yarinda Sunthornrangsri 1:01.29 | | 4. |  Morocco | 4:10.91 | | | 5 | 21. | | | Lina Khiyara 1:09.19, Imane El Baroudi 1:14.80, Samy Boutouil 54.53s, Souhail Hamouchane 52.39s | | 5. |  Mongolia | 4:15.32 | | | 7 | 23. | | | Enkh-Amgalangiin Ariuntamir 1:08.56, Zandanbal Gunsennorov 1:08.74, Batbayaryn Enkhkhüslen 1:04.98, Batbayaryn Enkhtamir 53.04s | | 6. |  Seychelles | 4:19.21 | | | 4 | 24. | | | Therese Soukup 1:11.96, Simon Bachmann 1:07.51, Mathieu Bachmann 57.36s, Khema Elizabeth 1:02.38 | | 7. |  Guam | 4:47.10 | | | 9 | 27. | | | Benjamin Ko 1:03.63, Keana Santos 1:35.53, Mia Lee 1:12.81, Israel Poppe 55.13s | | . |  South Korea | DSQ | | | 8 | -. | | | Lee Ju-ho 54.40s, Choi Dong-yeol 1:00.13, Kim Seo-yeong, Jeong So-eun | | -. |  Uganda | DNS | | | 3 | -. | | | | | -. |  Tanzania | DNS | | | 6 | -. | | | |
| Heat 1 | | time | | gap | L | Rk. | | 1. |  Hong Kong | 3:54.28 | | | 5 | 13. | | | Stephanie Au 1:01.38, Adam Chillingworth 1:02.16, Nicholas Lim 54.27s, Camille Cheng 56.47s | | 2. |  Vietnam | 4:07.53 | | | 4 | 19. | | | Nguyễn Quang Thuấn 59.87s, Phạm Thanh Bảo 1:02.69, Lê Thị Mỹ Thảo 1:03.76, Võ Thị Mỹ Tiên 1:01.21 | | 3. |  Northern Mariana | 4:37.91 | | | 3 | 26. | | | Juhn Tenorio 1:01.77, Maria Batallones 1:24.00, Taiyo Akimaru 1:02.22, Jinie Thompson 1:09.92 | | 4. |  Maldives | 4:58.06 | | | 6 | 28. | | | Aishath Sausan 1:18.58, Mubal Azzam Ibrahim 1:19.87, Mohamed Aan Hussain 1:07.09, Hamna Ahmed 1:12.52 |
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