The Merlin actress will be joined by fellow new additions Judy Greer and Lauren Lapkus in the fourth Jurassic Park movie.
Details about the trio's characters have yet to be revealed.
Chris Pratt and Bryce Dallas Howard will star in the Universal project from director Colin Trevorrow.
Nick Robinson, Ty Simpkins, Jake Johnson, Omar Sy, Irrfan Khan, Vincent D'Onofrio and BD Wong will also be among the supporting cast.
McGrath is best known for playing Morgana in BBC One's Merlin, while she currently stars as Lucy Westenra in NBC's Dracula.
Greer will also star in Dawn of the Planet of the Apes and Brad Bird's Tomorrowland.
Lapkus currently appears in Netflix's Orange is the New Black, and will soon be seen in Adam Sandler's Blended.
Jurassic World is scheduled for release on June 12, 2015.