The new recruits will make their on-screen debut in the BBC hospital drama - now in its 25th year - in late August as newly-qualified nurses Lloyd and Scarlett .
Obiora, who will play cocky new nursing recruit Lloyd, most recently appeared as receptionist Ben Trueman in axed BBC drama Hotel Babylon. The 25-year-old's other credits include Grange Hill and EastEnders, where he briefly appeared as the footballer boyfriend of former resident Chelsea Fox.
Meanwhile, Mantock, who is fresh of out ArtsEd drama school after studying on a bursary from the BBC Performing Arts Fund, will take on the role of terrified ingénue nurse Scarlett.
Joining Casualty's overworked emergency department, both Lloyd and Scarlett soon find themselves in the thick of the drama. Viewers will find that, despite his bravado, Lloyd's heart is in the right place. Although his tentative new colleague is horror struck after witnessing her first cardiac arrest, Scarlett soon proves her worth with a troubled teenage patient.
"We are delighted to welcome Michael and Madeleine to Casualty," says series producer Oliver Kent. "Michael is an established young star who brings a great swagger to the role of Lloyd and is sure to be a hit with our younger viewers, and Madeleine is an exciting new talent who is already proving herself alongside the more experienced heavy-hitters in the Casualty cast".
After teasing the arrivals of the "phenomenal" Obiora and "incredible" Mantock, Kent told Digital Spy that a new doctor will also be arriving "a bit further along the line".
In May, it was announced that ex-Hollyoaks star Gemma Atkinson and actor Dhafer L'Abidine will play new paramedics Tamzin and Omar. They will appear on-screen from September.
> Read our interview with Casualty producer Oliver Kent
> More exclusive character teasers from Casualty producer Oliver Kent