Below is a list of runners who have reached 100 marathons in 2021 along with the event and date this milestone was reached.
| Name | Event | Date |
| Michael White | Manchester To Liverpool 50M | 02-Apr-21 |
| Chris Mcfaul | Enigma Easter | 05-Apr-21 |
| Cathy Holroyd | Phoenix SG100 | 18-Apr-21 |
| Mick Entwistle | SVN Olympic Challange | 01-May-21 |
| Susan Cartwright | Two Castles | 02-May-21 |
| Simon Hogg | Hardmoors Wainstones | 09-May-21 |
| Julia Ebbrell | Worcester | 16-May-21 |
| Karen Hamilton | ZigZag Monkey Meander | 16-May-21 |
| Caroline Hunt | Phoenix Runderbird 1 | 27-May-21 |
| Laura Watts | SVN Relativity Run | 30-May-21 |
| Dean Sykes | SVN Relativity Run | 30-May-21 |
| Richard Glister | SVN Fowlmead 50 | 05-Jun-21 |
| Loretta Daley | Dartmoor Discovery | 05-Jun-21 |
| John Kennedy | Phoenix God of Thunder | 05-Jun-21 |
| Iain Leighton | Phoenix Wrath of Poseidon | 06-Jun-21 |
| Jamie Goodhead | Xnrg Pilgrims Way Day2 | 13-Jun-21 |
| Richard Boese | Saturn Summer Pop-Up Run | 13-Jun-21 |
| Ian Grange | Richmond Park | 13-Jun-21 |
| Stephen Hayes | Snickerthon | 26-Jun-21 |
| Lynne Maughan | SITC Dawn to Dusk | 27-Jun-21 |
| Simon Maughan | SITC Dawn to Dusk | 27-Jun-21 |
| Dean Sykes | SVN Olympic Run | 03-Jul-21 |
| Margaret Hodges | CODrc Independence Day | 04-Jul-21 |
| Emma Davis | Race To The Stones Day 1 | 10-Jul-21 |
| Ghanaya Talewar | SVN Smileathon | 11-Jul-21 |
| Katie Simpson | Phoenix Top Run II | 16-Jul-21 |
| Liz Peel | SVN Cone Of Doom | 17-Jul-21 |
| Keith Johnstone | IGUN Yorkshire Day | 01-Aug-21 |
| Brian Fields | Dorset Ooser | 08-Aug-21 |
| Cat D'Ascendis | Phoenix Mars Evening Run | 12-Aug-21 |
| Nicola Wilkins | WSR Summer Larmer | 31-Jul-21 |
| Jim Vince | Shere 42K Trail | 21-Aug-21 |
| David Rogers | SVN Zodiac Leo Challenge | 21-Aug-21 |
| Sue Jennings | Hardmoors 55 | 21-Aug-21 |
| Michael Bennett | Gatliff | 22-Aug-21 |
| Quentin Brocklebank | SVN Zodiac Aquarius Challenge | 27-Aug-21 |
| Simon Williamson | ADVA 100in10 Run Fest | 04-Sep-21 |
| Darren Heath | St Albans Stampede | 04-Sep-21 |
| John Nelms | St Albans Stampede | 04-Sep-21 |
| John Kinchen | Antwerp | 11-Sep-21 |
| Ian Merrick | Bacchus | 19-Sep-21 |
| Louise Lyon | SVN Zombie Challenge | 25-Sep-21 |
| Pete Brennan | Royal Windsor River Trail Run | 03-Oct-21 |
| Russell Harper | GRIM Slaithwaite Northern Jumble | 16-Oct-21 |
| Ammon Piepgrass | Autumn 100 | 16-Oct-21 |
| Mark Walker | Belfast Running Festival 24 hour | 16-Oct-21 |
| Michelle Double | Chelmsford | 17-Oct-21 |
| Damian Mcintyre | Lisbon | 17-Oct-21 |
| Paul Cooper | Queens of the Castle | 21-Oct-21 |
| Marie Debont-Booth | SVN 500 | 23-Oct-21 |
| Andrew Ellwood | Langdale | 23-Oct-21 |
| Karen Stanley | Newport | 24-Oct-21 |
| Debbie Camp | Hellstone | 24-Oct-21 |
| Chris Denton | EAMS Oktoberfest | 24-Oct-21 |
| Roz Glover | Saturn 10 Bees 10in10 Day 6 | 27-Oct-21 |
| Steve Marshall | 25 Hour Track 100 mile | 30-Oct-21 |
| Gemma Mallett | November Nightmare Day 6 | 05-Nov-21 |
| Robert Smith | Enigma Fireworks Day 3 | 06-Nov-21 |
| Dawn Gardner | Centurion Wendover Woods 50 | 06-Nov-21 |
| James Braithwaite | Phoenix From Hull & Back James' 100th | 06-Nov-21 |
| Charlotte Davey | Xnrg Druids Challenge Day 1 | 12-Nov-21 |
| David Heal | Phoenix Riverside | 21-Nov-21 |
| Mark Rouse | Shillington Shuffle | 21-Nov-21 |
| Tinu Ogundari | Tissington Trail | 21-Nov-21 |
| Giles Matsell | Big Bear Big Foot | 25-Nov-21 |
| Matthew Robins | Phoenix Christmas Advent Run | 01-Dec-21 |
| Michael Warner | Phoenix Christmas Advent Run | 01-Dec-21 |
| Richard Holden | SVN Cone Of Doom | 05-Dec-21 |
| Simon Ballard | SVN Festive Frolic | 18-Dec-21 |
| Nathalie Rees | CODrc Dawn to Dusk | 27-Dec-21 |
| Joanne Glister | SVN Saxon Shore 10th Anniversary Challenge | 28-Dec-21 |
| Leah Piper | SVN Planner Challenge | 31-Dec-21 |